Multiwallet sync progress reporting

[itswisdomagain]

Re-reading the sync progress notification code highlighted the following 2 issues:

• Stage 1 (headers fetching) gets done for all wallets before progressing to stage 2 whereas stage 2 (address discovery) does not get done for all wallets before proceeding to stage 3. After stage 1 is completed for all wallets, the first wallet goes through stages 2 and 3 before the next wallet goes through stages 2 and 3. The implication of this reporting is that the client app user interface shows progress reports for stage 2, then stage 3 (for 1 wallet), back to stage 2, then stage 3 again (for the next wallet) and so on until the last wallet.

• Overall sync ETA calculation currently appears to be done on a per-wallet basis rather than for all wallets. The implication of this would be a constantly changing estimated completion time on the UI.