MB-23714: Make VBucketPtr a std::shared_ptr

MB-23714: Make VBucketPtr a std::shared_ptr

Change the VBucketPtr definition so that VBucket poiners are managed
using std::shared_ptr.

To enable some functions to turn this* into a VBucketPtr, VBucket
inherits std::enable_shared_from_this. The only current user of this
is EphemeralVBucket where it constructs the DCPBackfillMemory with a
shared pointer to itself. Arguably EphemeralVBucket could be the class
to inherit std::enable_shared_from_this, but giving the base-class the
shared_from_this method seems more flexible for future enhancement.

Change-Id: Id2f39ece3983509b5c6742107de56b1dcba2d844
Reviewed-on: http://review.couchbase.org/76557
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>