first integration with go-couchbase for UPR.

- implemented KVFeeder interface for go-couchbase.
- re-use the same go-couchbase bucket instance for all feeds.
- changed from GetFailoverLog() API to GetFailoverLogs() API for go-couchbase.
- start using memcached.UprEvent structure instead of locally defined
  MutationEvent structure.

Pending issues,

- bucket instance need to be refreshed before computing VBServerMap().
- UprStreamRequest should be morphed as UprStreamBegin.
  Vbuuid field from STREAM_REQ should be sent downstream.
- UprStreamEnd need to be sent downstream.

long term issues - to handle the following opcodes,
  UprAddStream, UprCloseStream, UprStreamRequest, UprStreamEnd
  UprSnapshot, UprExpiration, UprFlush, UprControl, UprBufferAck
  UprNoop.

Change-Id: If2a213e8e535a61a8bb1545b5c62b5fc1ea2148a
Reviewed-on: http://review.couchbase.org/38476
Reviewed-by: Manik Taneja <manik@couchbase.com>
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>