Summary:
Prior to this diff it was easy for us to create too many IMAP connections (e.g.
by requesting many attachments at once), causing random failures when the
server would reject our connection attempts. This diff adds a per-Account IMAP
pooling mechanism so that we avoid these failures.
Test Plan:
Run locally with sync worker and several other clients using the
pool, verify correct behavior. Also added a few unit tests.
Reviewers: evan, spang, juan
Reviewed By: juan
Differential Revision: https://phab.nylas.com/D3965
