JVMCBC-51: Move Observe utility class from java-client

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 class PersistTo and ReplicateTo enums from
java-client to core-io.

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

Blocks: http://review.couchbase.org/42363
Change-Id: I649a17202234a912222a3c52eab141feb141a8cd
Reviewed-on: http://review.couchbase.org/42362
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Simon Baslé <simon@couchbase.com>