[3.0.1 backport] crc32c values with leading zeros trigger unnecessary import

Documents with matching crc32c values should not be imported by Sync Gateway.  We're using the server's crc32c macro to populate the values in _sync.crc32c - this pads leading zeros.  Sync Gateway's computation of the string representation of the crc32c value does not.

https://github.com/couchbase/sync_gateway/blob/master/base/util.go#L1060