connection_pool: introduce connection pool

Introduce ConnectionPool class to work with cluster of Tarantool
instances. ConnectionPool support master discovery and ro/rw-based
requests, so it is most useful while working with a single replicaset of
instances. ConnectionPool is supported only for Python 3.7 or newer.
Authenticated user must be able to call `box.info` on instances.

ConnectionPool updates information about each server state (RO/RW)
on initial connect and then asynchronously in separate threads.
Application retries must be written considering the asynchronous nature
of cluster state refresh. User does not need to use any synchronization
mechanisms in requests, it's all handled with ConnectionPool methods.

ConnectionPool API is the same as a plain Connection API.
On each request, a connection is chosen to execute this request.
A connection is chosen based on a request mode:
* Mode.ANY chooses any instance.
* Mode.RW chooses an RW instance.
* Mode.RO chooses an RO instance.
* Mode.PREFER_RW chooses an RW instance, if possible, RO instance
  otherwise.
* Mode.PREFER_RO chooses an RO instance, if possible, RW instance
  otherwise.
All requests that are guaranteed to write (insert, replace, delete,
upsert, update) use RW mode by default. select uses ANY by default. You
can set the mode explicitly. call, eval, execute and ping requests
require to set the mode explicitly.

Example:

  pool = tarantool.ConnectionPool(
      addrs=[
          {'host': '108.177.16.0', 'port': 3301},
          {'host': '108.177.16.0', 'port': 3302},
      ],
      user='test',
      password='test',)

  pool.call('some_write_procedure', arg, mode=tarantool.Mode.RW)

Closes #196

Author: DifferentialOrange

Diff for: CHANGELOG.md

@@ -9,6 +9,44 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Reusable testing workflow for integration with tarantool artifacts
   (PR #192).
+- Connection pool with master discovery (PR #211, #196).
+
+  ConnectionPool is supported only for Python 3.7 or newer.
+  Authenticated user must be able to call `box.info` on instances.
+
+  ConnectionPool updates information about each server state (RO/RW)
+  on initial connect and then asynchronously in separate threads.
+  Application retries must be written considering the asynchronous nature
+  of cluster state refresh. User does not need to use any synchronization
+  mechanisms in requests, it's all handled with ConnectionPool methods.
+
+  ConnectionPool API is the same as a plain Connection API.
+  On each request, a connection is chosen to execute this request.
+  A connection is chosen based on a request mode:
+  * Mode.ANY chooses any instance.
+  * Mode.RW chooses an RW instance.
+  * Mode.RO chooses an RO instance.
+  * Mode.PREFER_RW chooses an RW instance, if possible, RO instance
+    otherwise.
+  * Mode.PREFER_RO chooses an RO instance, if possible, RW instance
+    otherwise.
+  All requests that are guaranteed to write (insert, replace, delete,
+  upsert, update) use RW mode by default. select uses ANY by default. You
+  can set the mode explicitly. call, eval, execute and ping requests
+  require to set the mode explicitly.
+
+  Example:
+  ```python
+  pool = tarantool.ConnectionPool(
+      addrs=[
+          {'host': '108.177.16.0', 'port': 3301},
+          {'host': '108.177.16.0', 'port': 3302},
+      ],
+      user='test',
+      password='test',)
+
+  pool.call('some_write_procedure', arg, mode=tarantool.Mode.RW)
+  ```
 
 ### Changed
 - **Breaking**: drop Python 2 support (PR #207).

Diff for: tarantool/__init__.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 # pylint: disable=C0301,W0105,W0401,W0614
 
+import sys
+
 from tarantool.connection import Connection
 from tarantool.mesh_connection import MeshConnection
 from tarantool.const import (
@@ -76,3 +78,8 @@ def connectmesh(addrs=({'host': 'localhost', 'port': 3301},), user=None,
 __all__ = ['connect', 'Connection', 'connectmesh', 'MeshConnection', 'Schema',
            'Error', 'DatabaseError', 'NetworkError', 'NetworkWarning',
            'SchemaError', 'dbapi']
+
+# ConnectionPool is supported only for Python 3.7 or newer.
+if sys.version_info.major >= 3 and sys.version_info.minor >= 7:
+    from tarantool.connection_pool import ConnectionPool, Mode
+    __all__.extend(['ConnectionPool', 'Mode'])