Bridge relayer improvements

[shaspitz]

The standard bridge relayer implementation could benefit from the following features/improvements prior to mainnet:

• Relayer tracks transfers using DB and marks transfers as complete in DB when the transferFinalized tx is included in a finalized destination block.
• If a reorg happens on the destination chain, the finalizeTransfer transaction would need to be retried. This means the relayer must monitor each finalize transfer tx and make sure it gets into the finalized block (not "latest"), ie. confirm the txes are not reorged out.
• Relayer shouldn't sync from block zero upon being restarted (DB helps with this).
• Relayer should use keystore for signer.
• Implement Backup RPCs for each chain
• Bridge metrics

[shaspitz]

CC @aloknerurkar this is my TODO w.r.t bridge from earlier this year, and may not yet be a comprehensive list

[aloknerurkar]

Completed #438