I managed to reproduce some deadlocks during repo fetching with virtual worker threads. One notable trigger was some _other_ repo failing to fetch, which seems to cause Skyframe to try to interrupt other concurrent repo fetches. This _might_ be the cause for a deadlock where we submit a task to the worker executor service, but the task never starts running before it gets cancelled, which causes us to wait forever for a `DONE` signal that never comes. (The worker task puts a `DONE` signal in the queue in a `finally` block -- but we don't even enter the `try`.)

I then tried various things to fix this; this PR is an attempt that actually seemed to eliminate the deadlock. Instead of waiting for a `DONE` signal to make sure the worker thread has finished, we now hold on to the executor service, which offers a `close()` method that essentially uninterruptibly waits for any scheduled tasks to terminate, whether or not they have started running. (@justinhorvitz had suggested a similar idea before.) To make sure distinct repo fetches don't interfere with each other, we start a separate worker executor service for each repo fetch instead of making everyone share the same worker executor service. (This is recommended for virtual threads; see https://docs.oracle.com/en/java/javase/21/core/virtual-threads.html#GUID-C0FEE349-D998-4C9D-B032-E01D06BE55F2 for example.)

Related: #22003