This fixes various race conditions and leaks related to the user's sync
and API event stream. It was possible for a sync/stream to begin after a
user was already closed; this change prevents that by managing the
goroutines related to sync/stream within cancellable groups.
Add missing Close calls.
Properly handle nil channel for `user.startSync`.
This patch also updated liteapi and Gluon to latest master and dev
version respectively.
There was a race condition internal to the go-smtp library.
In order to fix it, a version bump was necessary.
However, this significantly changed the library interface.
