MB-10057: made pre-2.5.1 incoming xdcr checkpointing ineffective

2.5.0 and previous releases have buggy checkpointing. We're
implementing new checkpointing protocol. Which is incompatible with
previous releases.

For previous release we have to pretend that their's checkpointing
protocol "works". Without actually saving checkpoints. Otherwise
previous releases would fail to replicate to 2.5.1+ at all.

Our approach is to simply do nothing on ensure_full_commit and to drop
_local/ docs update on the floor, pretending that it actually
succeeded.

Change-Id: Ib495d3862c810b95c90c585dc7c1c5e79985200d
Reviewed-on: http://review.couchbase.org/34035
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10057: extracted capi_frontend:verify_bucket_auth

In order to make auth checks from other parts of capi layer.

Change-Id: Icd91a0e394d5e114ee52bbd6dbae1fa8beee13c6
Reviewed-on: http://review.couchbase.org/34036
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10057: added ns_memcached api for xdcr checkpointing ops

Notably we're adding new implementation of
wait_for_checkpoint_persistence which is using pool of "very long
call" memcached sockets and not usual path via ns_memcached workers.

It's different than plain wait_for_checkpoint_persistence because we
expect greater concurrency of xdcr wait_for_checkpoint_persistence
requests. Also xdcr variant handles retries and externally specified
timeout which normal variant does not. And for 2.5.1 I want to avoid
changing "normal" code.

Change-Id: I5c05b3214fd1ec661bbecaee1bbe45fcdc712fa7
Reviewed-on: http://review.couchbase.org/34037
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>