On startup sometimes language python worker crashes the process.

Repro steps:

```
mkdir py-func
cd py-func
python3 -m venv venv
source venv/bin/activate
func init . --worker-runtime python
pip install azure-functions==1.0.0a4 azure-functions-worker==1.0.0a4 grpcio==1.14.2 grpcio-tools==1.14.2 protobuf==3.6.1 six==1.11.0 tensorflow==1.5.0 pillow==5.0.0 numpy==1.14.1 flask urllib3
func new --name "Http Trigger"
func start
```

It usually works, but every now and then it'll crash with this exception:

```
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
at System.Collections.Generic.Queue`1.Enumerator.MoveNext()
at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
at System.String.Join(String separator, IEnumerable`1 values)
at Microsoft.Azure.WebJobs.Script.Rpc.LanguageWorkerChannel.OnProcessExited(Object sender, EventArgs e) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Rpc\LanguageWorkerChannel.cs:line 184
at Microsoft.Azure.WebJobs.Script.Rpc.LanguageWorkerChannel.b__34_2(Object sender, EventArgs e) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Rpc\LanguageWorkerChannel.cs:line 154
at System.Diagnostics.Process.OnExited()
at System.Diagnostics.Process.RaiseOnExited()
at System.Diagnostics.Process.CompletionCallback(Object waitHandleContext, Boolean wasSignaled)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
```

Start with src/WebJobs.Script/Rpc/LanguageWorkerChannel.cs around lines 154 and 184, then run the documented Python worker reproduction. Trace the process-exit handling and the reported collection-enumeration exception. Done means the startup scenario no longer crashes the host with this exception.

Written by the indexing model from the issue text.