While auditing the codebase for incorrect usage of constness, we found several places where fixing issues would break the API.

Methods incorrectly marked as const:

• mongocxx::client::list_databases() - calls mongoc_database_has_collection, which takes a non-const pointer to a mongoc_database_t
• mongocxx::collection::list_indexes() - connects to the server through libmongoc, which is not threadsafe
• mongocxx::insert_many_builder::insert() - non-threadsafe and will yield different results when called subsequently on the same collection

Additionally, the getter mongocxx::options::bulk_write::bypass_document_validation() should return a const ref, as the other getters return const refs.