More granular request process tracking (#330)

## Description of Changes
Addresses
clockworklabs/SpacetimeDBPrivate#1786 and
#329.
Marks messages as parsed from the background parsing thread, and then
marks them as applied using a separate tracker once they are applied.

## API

 - [ ] This is an API breaking change to the SDK

*If the API is breaking, please state below what will break*

## Requires SpacetimeDB PRs

## Testsuite

SpacetimeDB branch name: master

## Testing

- [x] Testing against Bitcraft

Author: kazimuth