do not mark tx as finalized until it is mined

[troggy]

EMM should only mark tx as finalized when corresponding exit tx is really landed into root chain.

Given:

1. Fast exit is submitted
2. EMM starts an exit transaction on a root network
3. exit transaction is failed for whatever reason

Expected:
transaction should not be marked as finalized in EMM database

Actual:
transaction is marked as finalized

Consequences:
when failed tx marked as processed, EMM won't retry exit and money will struck in exit handler.

Manual workaround:
to push stuck txs through, set finalized attribute to 0 for corresponding database record