Add caching to PythonExecutionService.getExecutablePath().

[ericsnowcurrently]

Every time PythonExecutionService.getExecutablePath() is called, it does a file stat and potentially an exec(). This is inefficient. It would make sense to instead cache the value returned from this function. ("this.executable"?) The value might need to be cached in the factory (if we aren't caching PythonExecutionService objects there already).

I suppose there is a risk that the file gets deleted later, causing hard-to-debug failures, but that is unlikely and already mostly an existing possible problem (which we never get complaints about...).

[karrtikr]

Did this at some point.