Worker pool for diffing off of the chainEvent channel

[i-norden]

Maintaining sequential order is no longer a priority, process the blocks off of the chainEvent chan concurrently using a statediff.Builder worker pool

e.g. here: https://github.com/vulcanize/go-ethereum/blob/v1.9.24-statediff/statediff/service.go#L196

This will require some rework of the lastBlock cache and the trie dereferencing to prevent race conditions.

[i-norden]

Satisfied in #43