Ensure IMAP connections get taken out of IDLE mode.

Summary:
This fixes an issue in which CrispinClient instances would get returned to the
connection pool while still in IDLE mode (e.g., if their containing greenlet
was killed). If reused in other contexts the connections could end up silently
hanging.

Fixes T688.

Test Plan:
Difficult to write a real regression test without committing account
credentials. But it does fix the issue on the problematic database dump.

Reviewers: siro, spang

Reviewed By: spang

Subscribers: spang

Maniphest Tasks: T688

Differential Revision: https://review.inboxapp.com/D924