[client-sync] Parse comma-separated References properly
[client-sync] Parse comma-separated References properly

This goes against the spec, but we've seen emails which use comma-separated
References instead of space-separated references. This could cause threading
to break if the message with comma-separated references was a reply and
we hadn't yet synced the message directly in the In-Reply-To header.

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