[enh] add `get_written_transactions_for_bma()` and `get_pending_transactions_for_bma()`

Closes #2

See merge request !2

[fix] rename `get_transactions_history_for_bma()` into 'get_txs_history_bma()'
[fix] rename `get_written_transactions_for_bma()` into 'get_tx_history_bma_written_by_blocks()' and change from/to into Option<u32>
[fix] rename `get_pending_transactions_for_bma()` into 'get_txs_history_bma_mempool()'

enh] add `get_written_transactions_for_bma()` and `get_pending_transactions_for_bma()` in DuniterModule trait - close #2