Fix ledger rpc getTransactions bug

[theochap]

Context

The PR #403 designs a series of fuzzing tests for ledger rpc using the protest API. A fuzzing test fails on CI (and not locally) probably due to concurrency issues.

How to reproduce the error

In test_rpc.rs in the test proptest_get_batches, the following test fails on CI but not locally

// TODO: Solve this test 
TestExpect{
       data: format!(r#"{{"jsonrpc":"2.0","method":"ledger_getBatches","params":[[{random_batch_num}], "Full"],"id":1}}"#),
       expected: format!(r#"{{"jsonrpc":"2.0","result":[{{"hash":"0x{batch_hash}","tx_range":{{"start":{first_tx_num},"end":{last_tx_num}}},"txs":[{tx_full_data}],"custom_receipt":{batch_receipt}}}],"id":1}}"#)},

Possible causes

It is pretty hard to track down the cause of failure for this test. The CI fails randomly a variable number of test cases, this test also randomly passes.
The failure is due to the fact that the list of transaction hashes expected does not match the list of transaction hashes delivered by rpc. The order of the transactions is modified and sometimes new hashes appear randomly to the transaction list.
It seems that this is probably a concurrency issue in the way rpc handles incoming transactions concurrently. The handlers and the code for the test are all mono threaded. It may also be an error related to CI configuration. This test sends a significant number of requests concurrently to RPC.

[neysofu]

Fixed by #443.