Create generic B-tree bucket DB and content node DB implementation

This is the first stage of removing the legacy DB implementation.
Support for B-tree specific functionality such as lock-free snapshot
reads will be added soon. This commit is just for feature parity.

Abstract away actual database implementation to allow it to
be chosen dynamically at startup. This abstraction does incur
some overhead via call indirections and type erasures of callbacks,
so it's likely it will be removed once the transition to the
new B-tree DB has been completed.

Since the algorithms used for bucket key operations is so similar
between the content node and distributor, a generic B-tree backed
bucket database has been created. The distributor DB will be rewritten
around this code very soon.

Due to the strong coupling between bucket locking and actual DB
implementation details, the new bucket DB has a fairly significant
code overlap with the legacy implementation. This is to avoid
spending time abstracting away and factoring out code for a
legacy implementation that is to be removed entirely anyway.

Remove existing LockableMap functionality not used or that's
only used by tests.

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