Add min parallelism and fix the build

[jolynch]

There are workloads which require more vertical scaling due to required parallelism, so now the planner can take read and write parallelism into account.

For example, imagine that a Java process creates 4 concurrent futures in response to a read request, then it needs at least 4 cores for that workload to run in parallel (as opposed to concurrent). The defaults are 1+1 for read+write which means our minimum instance size is effectively 2 cores. Given that hyperthreading is the default for most AWS instances this is reasonable.

Python 3.7 is EOL so remove testing that and add in Python 3.10.