Check packet events have not been handled before processing

[ancazamfir]

Closes: #983

Description

• check that a send packet has not been handled before processing
  • checks that the packet has not been received on the destination chain and
  • the packet commitment is still present on source chain (i.e. the packet was not timed out)
• check that a packet received and acknowledged on destination chain has not been already handled
  • checks that an acknowledgment packet has not been received by the source chain

(ignore the changes in the client worker...they are coming from PR #974 and will not be committed)

Testing:

• manually tested running multiple relayers with different accounts. Send a number of packets with high and small timeout offsets
• slower instance retries only once before already handled (found in logs) event is detected
• performed the tests for recv, ack and timeout packets

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[adizere]

Left some minor comments. I am still testing this..