Implement leapfrogging

[nopara73]

If the user is 100% sure it didn't receive transaction while it was away, it can skip the syncing.

[nopara73]

Another hybrid idea:

Download headers from the creation of the wallet (-100 headers) from trusted server.
Always check the last 100 header hash for reorgs.
Get notification for newblock from trusted server.
Download full blocks whenever wallet is online.
Mempool txs should come from trusted server.
From peers. Compare to headers.
If wallet closed with unconf txs download full blocks until it either confirms or 1008 block is downloaded.
If tx comes when wallet is off then user must manually write txid in it, it request over tor for trusted server then download full block.

Full wallet resyc: slow download all blocks

[nopara73]

In the new version this is not needed, it's light by design.