Merge 'Coroutinize commit log' from Calle Wilund

No real refactoring, just move the various methods to coroutines. Because coroutines are neat.
Broken down into one method per change to make review easier. And hoping I get tipped per change.

Grand idea being that using coroutines will eventually make real refactoring easier.
Unit tests + relevant dtest.

As discussed below, simply coroutinizing the code will, at least in the fast path, cause the slightly naive
compiler to generate multiple unused coroutine frames, dropping raw performance a bit.
The last two patches in this series addresses this, by breaking the fast path into non-coroutine
subroutines (no futures involved) and one coroutine main loop.

Results, as collected by `perf_simple_query` are:
Master (before changes):
```
{
        "parameters" :
        {
                "concurrency" : 100,
                "concurrency,partitions,cpus,duration" : "100,10000,1,30",
                "cpus" : 1,
                "duration" : 30,
                "partitions" : 10000
        },
        "stats" :
        {
                "allocs_per_op" : 52.237303521776113,
                "instructions_per_op" : 47403.34422198555,
                "mad tps" : 670.12528706749436,
                "max tps" : 140817.0800358199,
                "median tps" : 139391.58369995767,
                "min tps" : 133663.0095463676,
                "tasks_per_op" : 13.189605506751203
        },
        "test_properties" :
        {
                "type" : "write"
        },
        "versions" :
        {
                "scylla-server" :
                {
                        "commit_id" : "1f51bc67fd",
                        "date" : "20210712",
                        "run_date_time" : "2021-07-13 10:26:46",
                        "version" : "4.6.dev"
                }
        }
}
```

This PR (coroutines + fast path optimization patches):
```
{
        "parameters" :
        {
                "concurrency" : 100,
                "concurrency,partitions,cpus,duration" : "100,10000,1,30",
                "cpus" : 1,
                "duration" : 30,
                "partitions" : 10000
        },
        "stats" :
        {
                "allocs_per_op" : 52.208628061750559,
                "instructions_per_op" : 47300.501878330339,
                "mad tps" : 707.70233700674726,
                "max tps" : 139618.0661493362,
                "median tps" : 137891.11290420164,
                "min tps" : 127551.83433347062,
                "tasks_per_op" : 13.172121395660733
        },
        "test_properties" :
        {
                "type" : "write"
        },
        "versions" :
        {
                "scylla-server" :
                {
                        "commit_id" : "1d4b6f50bd",
                        "date" : "20210719",
                        "run_date_time" : "2021-07-19 09:27:09",
                        "version" : "4.6.dev"
                }
        }
}
```

I.e. both allocations/op and instruction count seem to be on par.

Closes #8954

* github.com:scylladb/scylla:
  commitlog: Make allocate_when_possible a template
  commitlog: break fast path alloc into non-fut/corout + outer loop
  commitlog: Drop stream/subscription from replayer
  commitlog: coroutinize commitlog::read_log_file
  commitlog: coroutinize commitlog::create_commitlog
  commitlog: coroutinize commitlog::add_entries
  commitlog: coroutinize commitlog::add_entry
  commitlog: coroutinize commitlog::add
  commitlog: change entry_writer usage to reference
  commitlog: coroutinize segment_manager::clear
  commitlog: coroutinize segment_manager::do_pending_deletes
  commitlog: coroutinize segment_manager::delete_file
  commitlog: coroutinize segment_manager::shutdown
  commitlog: coroutinize segment_manager::shutdown_all_segments
  commitlog: coroutinize segment_manager::sync_all_segments
  commitlog: coroutinize segment_manager::clear_reserve_segments
  commitlog: coroutinize segment_manager::active_segment
  commitlog: coroutinize segment_manager::new_segment
  commitlog: coroutinize segment_manager::allocate_segment
  commitlog: coroutinize segment_manager::rename_file
  commitlog: coroutinize segment_manager::init
  commitlog: coroutinize segment_manager::list_descriptors
  commitlog: coroutinize segment_manager::replenish_reserve
  commitlog: coroutinize segment::shutdown
  commitlog: coroutinize segment::close
  commitlog: coroutinize segment::batch_cycle
  commitlog: coroutinize segment::do_flush
  commitlog: coroutinize segment::flush
  commitlog: coroutinize segment::cycle
  commitlog: coroutinize allocate_when_possible
  commitlog: coroutinize segment::allocate