Request cancellation for language server

[L0r3m1p5um]

This implements client cancellation of language server requests via the $/cancelRequest notification. There's a limitation on what requests can be canceled. The request has to be waiting in the queue. If handling of the request has already begun, that request will be completed normally even if a cancellation notification is received. The server will never proactively cancel a request, this only cancels requests when requested by the client.

[github-actions]

Bencher Report

Branch	request-cancellation
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	microseconds (µs)
diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	11,064.00 µs
diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	188,620.00 µs
diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	381.35 µs
diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	533.75 µs
fibonacci 10	📈 view plot 🚷 view threshold	222.14 µs
foldl arrays 50	📈 view plot 🚷 view threshold	377.84 µs
foldl arrays 500	📈 view plot 🚷 view threshold	2,694.90 µs
foldr strings 50	📈 view plot 🚷 view threshold	2,906.40 µs
foldr strings 500	📈 view plot 🚷 view threshold	27,676.00 µs
generate normal 250	📈 view plot 🚷 view threshold	14,846.00 µs
generate normal 50	📈 view plot 🚷 view threshold	1,000.50 µs
generate normal unchecked 1000	📈 view plot 🚷 view threshold	1,675.10 µs
generate normal unchecked 200	📈 view plot 🚷 view threshold	345.19 µs
init-diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	55,208.00 µs
init-diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	209,370.00 µs
init-diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	49,990.00 µs
init-diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	50,306.00 µs
pidigits 100	📈 view plot 🚷 view threshold	1,912.90 µs
pipe normal 20	📈 view plot 🚷 view threshold	594.24 µs
pipe normal 200	📈 view plot 🚷 view threshold	4,477.60 µs
product 30	📈 view plot 🚷 view threshold	257.03 µs
requests-benches/inputs/goto-perf.ncl-000	📈 view plot 🚷 view threshold	3,021.30 µs
requests-benches/inputs/large-record-tree.ncl-000	📈 view plot 🚷 view threshold	613,440.00 µs
requests-benches/inputs/large-record-tree.ncl-001	📈 view plot 🚷 view threshold	116.20 µs
scalar 10	📈 view plot 🚷 view threshold	518.27 µs
sum 30	📈 view plot 🚷 view threshold	268.28 µs

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