Support for setting up expiry with subdoc mutation operations

Updating document's ttl as part of subdoc mutation doesn't seem to work. Attached standalone program to replicate the issue.

Sample run:

➜  ./test "couchbase://127.0.0.1:12000/default?username=eventing&select_bucket=true"

Operation failed rc: 3a (The operation structure contains conflicting options)

Assertion failed: (rc == LCB_SUCCESS), function main, file xattr.cc, line 143.

[1]    18134 abort      ./test

It will be useful if this could be addressed in next release iteration of lcb. We(in eventing) depend on it to restore document's metadata in cases where only xattr is updated.