L2: improve and refactor l1_watcher

[fborello-lambda]

The l1_watcher requires some changes.
When the proposer is restarted, the l1_watcher processes all the events, resulting in duplicate deposit transactions.
This behavior was observed when depositing the same amount twice. It may be related just to Issue #1191, or perhaps additional logic is required.

TaskList:

• fix(L2): add uniqueness to deposit logs on L1 #1191