Fixes Issue 1643

Fixes Issue 1664

Issue 1643: Some servers respond with an untagged EXISTS to every
NOOP.  This change parses through all untagged responses in a loop
until no more are being generated, avoiding calling internalOpen each
time.

Issue 1664: I have never personally seen this error, and the log looks
like a thread safety problem.  mMessageCount is being set in one
thread (the IDLE thread) but not properly visible in another thread
(the check mail thread).  mMessageCount and other ImapFolder private
variables are now volatile in hopes of correcting this problem.