Use a mutex to lock the sync transaction, uniquely identified by the remote
folder name.
Serializing the syncs prevents any IDLE thread from syncing the same folder of
a concurrent autorefresh thread. Othewise, we could download new messages
twice, or read "one message past the last one" which in some cases creates a
duplicated messages. Also prevents IDLE threads from reading new messages
before the autorefresh thread finishes synchronization.
Github-ref: #421
Originally-submitted-by: Michael Hohmuth <hohmuth@sax.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>
fc079499e9