Add site config `debug.search.symbolParallelism` option
Created by: slimsag
This adds a site config option to debug the amount of parallelism we use when performing symbol searches. This is theorized to be an underlying cause of #5058 and as such I am adding this option to rule it out. In the future, this option will be removed.
Helps #5058
Test plan: existing tests, no functionality change except when using new option.