[client-sync] Refactor raw message IMAP connection pool use
[client-sync] Refactor raw message IMAP connection pool use

Summary: In preparation for removing timeout handling from IMAPConnectionPool.

Test Plan: Run locally

Reviewers: juan

Reviewed By: juan

Differential Revision: https://phab.nylas.com/D4168

nirmit/nylas-mail,nylas-mail-lives/nylas-mail,nylas-mail-lives/nylas-mail,nirmit/nylas-mail,nylas-mail-lives/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,nylas-mail-lives/nylas-mail,nylas/nylas-mail,nylas-mail-lives/nylas-mail,nirmit/nylas-mail,simonft/nylas-mail,nylas/nylas-mail,simonft/nylas-mail,simonft/nylas-mail,nirmit/nylas-mail,nirmit/nylas-mail,nylas/nylas-mail,nylas/nylas-mail,nylas/nylas-mail