Make SetFullPath no-op in WAL transaction

SetFullPath RPC writes the repository's full path into the git config. This is not supported by WALtransaction. That RPC was scheduled for removal in some next versions. Until then, we should mark that RPC as a no-op when being executed in the context of a WAL transaction.

This MR returns Unimplemented status code if the RPC handler detects an active WAL partition manager. From the definition, Unimplemented status code is for "The operation is not implemented or is not supported/enabled in this service". That's a perfect status code for this purpose.