RPCDaemon: reduce --rpc.batch.concurrency default from 50 to 2

[AskAlexSharov]

Reason:

• benchmark of large batches with fast sub-requests showed that high concurrency harming it.
• benchmark of much parallel batches also suffer from too high parallelism.
• value 2 still increasing throughput of batches with fast sub-requests 2 times. (value 4 increasing throughput 3 times, which is not great anymore, then further increasing must be done manually depending on traffic and server capacity).
• value 2 useful when batch has much sub-requests, but small sub-set of them is slow.
• users who have much slow sub-requests will need manually increase it - depend on their Server capacity.