Add a new option to RestoreBkpBlocks() to indicate if a cleanup lock should

Add a new option to RestoreBkpBlocks() to indicate if a cleanup lock should
be used instead of the normal exclusive lock, and make WAL redo functions
responsible for calling RestoreBkpBlocks(). They know better what kind of a
lock they need.

At the moment, this just moves things around with no functional change, but
makes the hot standby patch that's under review cleaner.