Deduplicate common concurrent RPC calls

[ptrus]

Some method calls are extremely common (mostly method calls for fetching latest block info).

Using a package like sync/singleflight these calls can easily can easily be de-deuplicated so that the DB/oasis-node is queried only once for any number of concurrent requests.

The following methods are obvious candidates:

• GetBlockByNumber - dedeuplicate by: {blockNumber}-{fullTx}
• GetBlockTransactionCountByNumber - deduplicate by: {blockNumber}
• GasPrice
• GetBlockHash - deduplicate by: {blockNumber}
• BlockNumber

Potentially (if overhead of using singleflight is small enough) this could be done for all read-only RPC calls.