With a large tablet count, e.g. 128k, forward_service::dispatch() can potentially stall when grouping ranges per endpoint.
` Reactor stalled for 4 ms on shard 1. Backtrace: 0x5eb15ea 0x5eb09f5 0x5eb1daf 0x3dbaf 0x2d01e57 0x33f7d1e 0x348255f 0x2d005d4 0x2d3d017 0x2d3d58c 0x2d3d225 0x5e59622 0x5ec328f 0x5ec4577 0x5ee84e0 0x5e8394a 0x8c946 0x11296f
`
Also there are inefficient copies that are being removed. partition_range_vector for a single endpoint can grow beyond 1M.
Closesscylladb/scylladb#18695
* github.com:scylladb/scylladb:
service: fix indentation in dispatch()
service: fix reactor stall with large tablet count
service: avoid potential expensive copies in forward_service::dispatch()
service: coroutinize forward_service::dispatch()
6982de6dde