This code was doing a sync dispatch to a queue it was already running
on. Instead, go through the same path as other close calls. This would
hang, or if you're lucky crash when dispatch detects the issue.
We implicitly close the connection when the input file descriptor is
closed, or if it has an error. But we also close things down explicitly
when we see the exit notification. We need to protect against a race
triggering us to call the close handler multiple times. This was
resulting in `sourcekit-lsp` racing to call `exit(0)`, which triggered a
double-free and abort on Linux.
When testing the sourcekit-lsp binary, it is handy to be able to force
requests to be handled synchronously. This only affects the protocol
layer, not the implementation. This option is hidden from the help text
and should only be used for testing/debugging.
