JCBC-604: Move Observe utility class to core-io

Motivation
----------
Each wrapper of core-io library have to implement Observe polling
feature. This way is more error prone and harder to maintain good
quality of this implementation, which in fact will be shared.

Modification
------------
Pull Observe utility classes from java-client to core-io. And wrap
PersistTo/ReplicateTo enums from core-io library with the ones from
java-client to preserve API compatibility

Result
------
Duplication reduced (considering java-client and jruby-client
libraries). Potentially expanded coverage for the Observe
implementation.

Depends-On: http://review.couchbase.org/42362
Change-Id: Ia2e7602f707a616655dd46f5db5718e674ba983c
Reviewed-on: http://review.couchbase.org/42363
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>