This should speed up synchronization / restoration of large wallets.
Wallets are written only when they switch to up_to_date state, or
when stop_threads() is called when closing the daemon, or when
a command line command finishes.
Proper fix for #1525.
Using python's GC module, I've verified that the daemon, when running,
now releases all verifiers, synchronizers and wallets - all the resources
we care about releasing.
The synchronizer's work is done from the network proxy's main loop.
A minor problem with the old synchronizer was that it considered itself
out of date if the network was out of date. This was too generic: the
network can have pending requests unrelated to the synchronizer. This
resulted in the synchronizer often unnecessarily flipping the wallet
between up-to-date and not-up-to-date, and causing unnecessary calls
to wallet.save_transactions(). This was observable when opening the
network dialog box: frequently just opening it would cause a wallet
status change and transaction flush, simply because the network dialog
sends a get_parameters() request. This rework of the synchronizer does
not have that issue.