Allow Gmail threads to be split among multiple Inbox threads.

Summary:
This is a short-term fix for an uncommon situation that nonetheless breaks
sync. See T599 for details.

Test Plan:
Trigger the breaking condition by sending multiple identical emails;
check that they're reconciled correctly, and that replies get threaded with the
message they're replying to.

Reviewers: spang, kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D713
-update my.cnf, dockerfile stuff now expects to be one directory level up
-clear out pyc files and chown source dir to admin in Dockerfile
-attempt to grab mysql host/port info from docker link env vars before falling back to config file
-Document docker env var naming conventions.
-Uncomment client base my.cnf