For newly created database - Make `start_block` dynamic instead of static

[dnwiebe]

When a database is created, initialize start_block to zero.

When the start_block is detected to be zero, do an early blockchain transaction to determine the block number of the head of the blockchain, and set start_block to that.

Blocked by #600

COMMENT - the start_block will likely not ever be zero (perhaps null as empty), since it would be populated by the blockheight of the token contract creation.

Test Plan

• Before starting a test. Backup your existing database if you want to use it again to start over.
• Start with no database to allow one to be created. Payments will only be found from the latest block onward. start_block will be initialized to null in the database and treated as none until the first Accountant receivables scan and will be updated to the latest block.
• An existing database should retain the existing block number if one exists.

[utkarshg6]

@dnwiebe I believe this might be helpful - https://docs.rs/web3/0.8.0/web3/types/struct.SyncInfo.html

We can use either of these two fields current_block or highest_block.

[kauri-hero]

One test to consider too, is if the very first RPC call to find the current blockheight fails, does Node panic or provide an error?

Bert - if this occurs in a scan, the scan may just abort and Node will continue running. Will likely run on the next interval. You would have to differentiate which scan is the one failing