Avoid double send for transactions when handle contract events

[akolotov]

Currently the bridge could send the same transactions several times if the database is not updated properly.

The following changes could be introduced:

1. To handle Deposit() event and avoid invocation of deposit() several times for the same origin transaction:

For Foreign chain it is necessary to check that

depositsSigned(sha3(ValidatorAddress, sha3(HomeDepositSender, Value, HomeTransactionHash)))

returns false and invoke deposit() in this case only.

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2. To handle Withdraw() event on the Foreign side and avoid invocation of submitSignature() several times for the same 'message'

For Foreign chain it is necessary to check that

messagesSigned(sha3(ValidatorAddress, sha3(message)))

returns false and invoke submitSignature() in this case only.

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3. To handle CollectedSignatures() event on the Foreign side and avoid invocation of withdraw() several times for the the same origin transaction

For Home chain it is necessary to check that

withdraws(ForeignTransactionHash)

returns false and invoke withdraw() in this case only.

[rstormsf]

very good explanation
one more stategy to this is to run estimateGas it should succeed

[akolotov]

my expectation is that estimateGas will require more time to check tx. But it needs to be measured first of course.