p2p: fan in incoming txns into backlog worker

[algorandskiy]

Summary

While investigating p2p TX traffic and performance, I found transaction pool mutex congestion. This PR is a PoC to use backlogWorker as a pool only accessor similarly to wsnet. Implementation summary:

• Added syncCh channel to work item wi
• When it is initiated from gossipsub validator, syncCh is initiated and awaited making validation synchronous as before.
• At each tx validation check backlogWorker checks if syncCh is set and responds with validation result to the channel.

Additionally, there are couple more fixes that helped with TPS as well:

• add some workers to pull sub.Next() faster.
• cache signed transaction ID to reuse in remember and txpool recomputing on a next block arrival.

Test Plan

1. Unit tests passed
2. Cluster TPS test showed +700-900 TPS gain (total 7.5k TPS) on a single txn payment scenario.