Investigate how to handle incoming transactions when the node is performing BlockSync or StateSync.

Tasks:

• The RFC discusses an optimization in the mempool reactor. Add a comment in the code with a reference to the RFC.

DoD:

• An RFC explaining the current implementation and discussing potential problems or improvements. rfc: incoming txs while catching up #735
• New issues resulting from the RFC, if any.
  • mempool: Reject incoming txs while catching up #785