datastore: LocalTransaction::first_txn_num_in_block: not yet implemented

Running standalone `rpcdaemon` at commit 95a6a2da8c3f2c303ac56cffe0d2f3a792af32bd using local E3 beta1 data storage, the following RPC integration test fails:
```
python3 ./run_tests.py --api-list eth_call --run-single-test 1 --dump-response

Runs tests in parallel
0411. http ::eth_call/test_01.json Failed
TEST ABORTED!

Test time-elapsed: 0:00:00.339054
Number of executed tests: 1/1156
Number of NOT executed tests: 0
Number of success tests: 0
Number of failed tests: 1
```
producing the following response diff:
```
{
- result: "0x"
+ error: {
+ code: -32603
+ message: "localtransaction::first_txn_num_in_block: not yet implemented"
+ }
}
```