Make evm creation for eth_call and debug_traceTransaction dynamic

[carneiro-cw]

Currently we create a fixed number of evms (based on a config param) for eth_call (past and pending) and debug_traceTransaction. We should make it so the config only sets a baseline of threads to be created on startup, but that can scale up to some limit based on the volume of calls (and scales back as the volume decreases).

stratus/src/eth/executor/executor.rs

Line 202 in 129ff91

max(config.executor_call_present_evms.unwrap_or(config.executor_evms / 2), 1),

[carneiro-cw]

I explored this a bit and if we want to always have some threads available there is only one threadpool library currently implemented in rust that gives us that option https://github.com/robinfriedli/rusty_pool. If growing/shrinking is not required we could use rayon's threadpool (or some other solution), or implement our own which shouldn't be too hard considering we kindof already have something similar.