Refactor for threadsafe BucketListDB

[SirTyson]

Description

First step towards #4127

This PR refactors BucketListDB code as a first step towards parallel BucketListDB access. This should have no observable changes.

Currently, BucketListDB is thread safe with the exception of Bucket file streams. This change refactors all BucketListDB code out of BucketList and Bucket and into SearchableBucketListSnapshot and SearchableBucketSnapshot. SearchableBucketListSnapshot is a lightweight wrapper around BucketList that stores SearchableBucketSnapshot. SearchableBucketSnapshot is another wrapper class that maintains a pointer to the underlying Bucket as well as a stream to read from the Bucket. In order to facilitate multi thread reads, all BucketList loads must go through these snapshot classes that maintain their own local stream object.

In addition to the refactor, a few mutexes have been added around adding entries to the BucketList and modifying ledger header pointers. While these locks are not currently necessary, further work will build on this and actually enable parallel access.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[SirTyson]

Closing, we should just merge #4176 instead.