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.
User used to connect 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 selected to execute this request.
Connection is selected based on request mode:
* Mode.RW selects an RW instance.
* Mode.PREFER_RW selects an RW instance, if possible, RO instance
  otherwise.
* Mode.PREFER_RO selects an RO instance, if possible, RW instance
  otherwise.
All requests that are guaranteed to write (insert, replace, delete,
upsert, update) use RW mode. For any other type of request (call, eval,
execute, ping, select) mode could be set explicitly.

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

Closes #196

Author: DifferentialOrange

Diff for: tarantool/__init__.py

@@ -80,3 +80,8 @@ 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'])