Allow user to specify number of workers

[garrettjstevens]

As discovered with @cmdcolin yesterday, there was a workerCount property in BaseRpcDriver that was set to 0 and had no way to change it, meaning the worker count was always auto-calculated. We also realized that the config for the worker was not getting passed to the worker driver. This fixes that by passing the worker configs to the drivers and then reading workerCount out of the config. You can specify a certain number of workers like this:

{
  "configuration": {
   "rpc": {
     "drivers": {
       "WebWorkerRpcDriver": {
         "type": "WebWorkerRpcDriver",
         "workerCount": 2
        }
      }
    }
  }
}

This also contains a bug fix related to #2798. As a consequence of that change, if you had a lot of tracks open when the page loaded, it would sometimes create some unused workers, but this PR fixes that.

[codecov]

Codecov Report

Merging #2829 (4bfa41e) into main (26a8881) will increase coverage by 0.00%.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##             main    #2829   +/-   ##
=======================================
  Coverage   59.98%   59.99%           
=======================================
  Files         584      584           
  Lines       26688    26695    +7     
  Branches     6462     6467    +5     
=======================================
+ Hits        16010    16016    +6     
- Misses      10350    10351    +1     
  Partials      328      328           

Impacted Files	Coverage Δ
packages/core/rpc/RpcManager.ts	70.58% <41.66%> (-15.13%)	⬇️
packages/core/rpc/BaseRpcDriver.ts	84.76% <88.00%> (ø)
packages/core/rpc/configSchema.ts	100.00% <100.00%> (ø)
products/jbrowse-web/src/Loader.tsx	63.84% <100.00%> (+0.28%)	⬆️
.../linear-genome-view/src/LinearGenomeView/index.tsx	85.27% <0.00%> (+0.17%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	80.80% <0.00%> (+3.03%)	⬆️
packages/core/TextSearch/TextSearchManager.ts	100.00% <0.00%> (+3.44%)	⬆️

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