MB-23637: Extra logging for dcp-vbtakeover stat & refactor stream type

MB-23637: Extra logging for dcp-vbtakeover stat & refactor stream type

The dcp-vbtakeover stat is critical to the success of rebalance.
Therefore it is useful for debugging future rebalance failures to log
the unexpected paths and error paths.

We want to print the stream type in the logging output.  Therefore the
stream type has been refactored into a enum class and a to_string
method provided to print a textual representation of the type.

Change-Id: I72ce7c6bd1f923f3cfa7557f1b150f97aa5fa7f9
Reviewed-on: http://review.couchbase.org/76162
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>