[Durability]Subdoc replace/remove over the sync_write locked document returns 'SYNC_WRITE_AMBIGUOUS'

Build: 6.5.0-4960

Scenario:

1. 2 node cluster, Couchbase bucket(replica=1)
2. Load one document 'doc_1'
3. New client is created to perform step#4 and Stop Memcached on replica node (kill -STOP)
4. Perform sub_doc insert for 'doc_1' from client-1 with timeout=60, durability=MAJORITY
5. Perform sub_doc replace/delete over the same doc 'doc_1' and same subdoc_path as step#4 using client-2

Observation:

Sub_doc replace/remove operation returns DurabilityAmbiguousException with reason: SYNC_WRITE_AMBIGUOUS

{u'xattrs-0': {'error': com.couchbase.client.core.error.DurabilityAmbiguousException: The server returned with a durability ambiguous response on this request {"status":"SYNC_WRITE_AMBIGUOUS"}, 'cas': 0, 'value': None}}

Expected behavior:

Since doc_1 is already under sync_write locked state, we should see 'AmbiguousTimeoutException' with reason 'KV_SYNC_WRITE_IN_PROGRESS' due to continuous retries configured from SDK side.

Note:

Marking this as regression since this case worked with 6.5.0-4947 using SDK-beta.1

As per 6.5.0-4947, expected sdk exception was 'TimeoutException' with reason 'KV_SYNC_WRITE_IN_PROGRESS'

Testcase:

subdoc.subdoc_xattr.SubdocXattrDurabilityTest.test_subdoc_sync_write_in_progress,nodes_init=2,replicas=1,sdk_timeout=60,xattr=False,durabiilty=MAJORITY