refactor(ingestor): WAL Replay controller: Use singleflight to ensure only one inflight flush is happening

[Segflow]

What this PR does / why we need it:
When going through the codebase to understand how we replaying the WAL etc.. I come across this interesting piece of code and I thought I could simplify it by using the singleflight package instead of current approach (complex sync.Mutex and sync.Cond)

Special notes for your reviewer:

The previous approach work just fine and passes the new test too. This is only to make things easier to reason about.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[Segflow]

Benchmark shows it's slightly faster (but negligeable) :

Before the change (using mutex and cond)

goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/ingester
cpu: AMD Ryzen 5 9600X 6-Core Processor             
BenchmarkReplayControllerBackpressure/without-backpressure-12         	 1134704	       978.1 ns/op	         0 total_flushes	       0 B/op	       0 allocs/op
BenchmarkReplayControllerBackpressure/with-backpressure-12            	    1066	   1108561 ns/op	         1.001 ops_per_flush	      1065 total_flushes	     905 B/op	      26 allocs/op
PASS
ok  	github.com/grafana/loki/v3/pkg/ingester	3.388s

After the change (using singleflight)

goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/ingester
cpu: AMD Ryzen 5 9600X 6-Core Processor             
BenchmarkReplayControllerBackpressure/without-backpressure-12         	 1313337	       896.6 ns/op	         0 total_flushes	       0 B/op	       0 allocs/op
BenchmarkReplayControllerBackpressure/with-backpressure-12            	    1050	   1125361 ns/op	         1.001 ops_per_flush	      1049 total_flushes	     696 B/op	      15 allocs/op
PASS
ok  	github.com/grafana/loki/v3/pkg/ingester	3.219s

[chaudum]

LGTM