improve wallet synchronization

[ecdsa]

synchronization of large wallets is slow.
synchronizaion could be separated in stages, with the existing protocol:

1. address generation. this requires only address.get_status
2. request UTXOs for each address
3. request transactions with UTXOs
4. request merkle branches
5. request full histories
6. request spent-only transactions
   the wallet balance can be displayed when 3 is completed.

[bauerj]

Once this is done, we can also show a progress bar to the user, indicating how far the synchronization has proceeded.

[SomberNight]

in the context of potential protocol changes to help this, tangentially related:
kyuupichan/electrumx#82
kyuupichan/electrumx#348