Adapt internal code to lcb 2.4

This changes the internals of the code to be more streamlined towards
libcouchbase 2.4. This in itself does not change any public APIs.

Change-Id: Icaa8560e3c3f9f66e0829cc33491a6debbaf5ce9
Reviewed-on: http://review.couchbase.org/41301
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Use lcb_destroy_async for asynchronous destruction

Change-Id: I733b78a9fae837a5ae0bb7926ff2f28828214bac
Reviewed-on: http://review.couchbase.org/41304
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Don't try to destroy instance on async connection if non exists

Change-Id: Ic962fa0e913062b47fe9e9926df6eabe7949c029
Reviewed-on: http://review.couchbase.org/41303
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>