[sync] "Download & transact into unchanged client" flow (upstream-only changes)

[grigoryk]

Given...

• an unchanged local client
• a downloading http client ([sync] HTTP client for downloading remote transactions #511)
• a transaction validator ([sync] Remote transaction validation #515)
• a local transaction write client ([sync] Local transaction write client #514)
• ability to assert datoms after inserting transactions ([sync] Raw transact #512)

... implement the following flow:

• Download any transactions after currently known remote_head
• Check sanity of downloaded transactions
• Process transactions through the local transaction writer
• Advance remote_head via SyncMetadataClient

[grigoryk]

Completed in #563.