Envelope can now only be accessed from within a RwLock. Two new structs
are introduced: EnvelopeRef and EnvelopeRefMut. These hold a reference
to an Envelope and the mutex guard that keeps them alive.
This change allows sharing of the envelopes hash map amongst threads.
Depending on the insertion order of folders which is non-deterministic
because it relies on the kernel's scheduling of parsing threads, the
listing the user sees might not be up-to-date because later thread
updates are never broadcast. This results in inconsistencies between
threads and mail listings when a thread's root envelope was part of a
not broadcast update leading to `key not found` panics in a listing's
hashmaps.
Manos Pitsidianakis