[clangd] Allow all LSP methods to signal cancellation via $/cancelRequest

Summary:
The cancelable scopes are managed by JSONRPCDispatcher so that all Handlers
run in cancelable contexts.
(Previously ClangdServer did this, for code completion only).

Cancellation request processing is therefore also in JSONRPCDispatcher.
(Previously it was in ClangdLSPServer).

This doesn't actually make any new commands *respect* cancellation - they'd
need to check isCancelled() and bail out. But it opens the door to doing
this incrementally, and putting such logic in common machinery like TUScheduler.

I also rewrote the ClangdServer class/threading comments because I wanted to
add to it and I got carried away.

Reviewers: ilya-biryukov, kadircet

Subscribers: ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D52004

llvm-svn: 342135