Log Message:
Update databases/mongo-c-driver to 1.9.0.

This version drops support for MongoDB 2.4 and adds support
for MongoDB 3.6 features:

- New struct mongoc_change_stream_t to watch a collection for changes.
- New struct mongoc_client_session_t represents a MongoDB 3.6 session,
  which supports causal consistency: you are guaranteed to read your writes
  and to perform monotonic reads, even when reading from secondaries or in
  a sharded cluster.
- New functions that accept flexible options as a BSON document. These
  accept a "sessionId" option and any future options. In addition, the
  two new "update" functions accept the "arrayFilters" \ 
option that is new
  in MongoDB 3.6:
   - mongoc_collection_insert_one
   - mongoc_collection_insert_many
   - mongoc_collection_update_one
   - mongoc_collection_update_many
   - mongoc_collection_replace_one
   - mongoc_collection_delete_one
   - mongoc_collection_delete_many
   - mongoc_client_command_with_opts
   - mongoc_database_command_with_opts
   - mongoc_collection_command_with_opts
   - mongoc_client_find_databases_with_opts
   - mongoc_client_get_database_names_with_opts
   - mongoc_collection_create_bulk_operation_with_opts
   - mongoc_collection_find_indexes_with_opts
   - mongoc_database_find_collections_with_opts
   - mongoc_database_get_collection_names_with_opts
- New URI option "retryWrites=true" safely and automatically retries \ 
certain
  write operations if the server is a MongoDB 3.6 replica set or sharded
  cluster.
- Support for MongoDB OP_MSG wire protocol.

Additional changes not specific to MongoDB 3.6:
- Support for mongodb+srv URIs to query DNS for SRV and TXT records that
  configure the connection to MongoDB.
- Support LibreSSL with CMake build
- The "minPoolSize" URI option is deprecated: it's confusing and not \ 
useful.

Bug fixes:
- mongoc_bulk_operation_execute did not always initialize "reply".
- Fix C99 pedantic warnings.