Summary:
The global type environment is loaded in all analysis workers, so it is best to minimise its memory footprint. `MaxSharing` does a great job at this but
- it is very slow
- it may be involved in observed segfaults, since it uses potentially unsafe operations

This diff replaces `MaxSharing` with a post-facto poor-man's hashcons pass through the parts involved in a java type environment. It is not as efficient as `MaxSharing` at reducing footprint, but it's much faster.

Reviewed By: jberdine

Differential Revision: D24590400

fbshipit-source-id: c37100325