minor: add address sanitizer builds to travis
CXX-20 add initial support for write commands
CXX-20 first round of improvements
CXX-20 changing the way some things work
CXX-20 first pass at multi DBClientWriter tests
CXX-20 stop exporting namespace-string headers
CXX-20 skip write command tests on unsupported server
CXX-20 fixups from last round of reviews
CXX-20 check for maximum sizes
CXX-20 additional tests for maximum size limits

Disabled for now until we can figure out a good way to synthetically
reduce maxBsonObjectSize for DBClients.
CXX-20 last minor fixups
CXX-20 windows hates ssize_t
CXX-20 more windows shenanigans
CXX-20 scoped_db_conn_test fixups