Improve feeReceivers script

[jacque006]

Problem

The feeReceiver script in #636 has a number of edge/reuse cases where it will not work as effectively as it could, including things like checking for an existing registered pubkey, generating only fee receivers for new registered tokens, etc.

Solution(s)

Reuse

• Check for an existing registration of the wallet public key in pubkey tree before registering a new one.
• Bootstrap the script off an existing feeReceivers config entry and only add newly registered tokens.
• Have the script bootstrap off of leveldb data or an already running Hubble node's RPC interface so it can more quickly find existing data without having to replay L1 chain data.

Other

• Use MassMigration or a new bulk version of DepositManager.depositFor to allow for more fee receiver states to be created in less L1 transactions.
• Add an optional cli flag to allow the script to generate filler deposits to speed up the packing of pending deposits. These filler deposits could be for tokenIDs that don't exist yet so a node operator already has them setup as new tokens are added.