This commit attempts to clarify which functions in the Db interface may be
returning cached data that the InvalidateCache, InvalidateBlockCache, and
InvalidateTxCache functions are used to clear.
The Db interface is intended to work with block heights as opposed to
specific database ids which may or may not be the same as the block
height. This commits changes the function names to make that distinction
a little more clear.
The function is being added back in since the tests rely on it, but it is
marked as DEPRECATED since it really should not be a part of the public
generic db interface.
