inhibitor merge cache and index so there's only one lock

[Spaceman1701]

This change replaces the store.Alerts with a new cache type in the inhibitor. The cache is essentially a merged store.Alerts and inhibitor index that work under a single lock. This allows operations to modify the cache and index atomically, solving a bunch of possible race conditions in the Inhibitor.

I believe that this resolves the last problems caught by the new tests added in #5449.

[BUGFIX]: Fix a race condition where the inhibitor could get into a bad state and fail to inhibit some alerts in some circumstances

[SoloJacobs]

❯ ./run.sh
=== inhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (4.79s)
ok  	github.com/prometheus/alertmanager/inhibit	4.799s
=== patchedinhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (4.93s)
ok  	github.com/prometheus/alertmanager/patchedinhibit	4.940s
=== v29inhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (10.24s)
ok  	github.com/prometheus/alertmanager/v29inhibit	10.254s
=== 5449updatedinhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (5.55s)
ok  	github.com/prometheus/alertmanager/5449updatedinhibit	5.556s
=== 5542updatedinhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (5.24s)
ok  	github.com/prometheus/alertmanager/5542updatedinhibit	5.250s
=== 5559updatedinhibit
flushes: 238231
Mutes calls: 297539
alerts muted: 201516 (67.73%)
flushes fully muted: 178227 (74.81%)
--- PASS: TestReplay (4.52s)
ok  	github.com/prometheus/alertmanager/5559updatedinhibit	4.527s

(inhibit test was rebased on main, so that is why this test no longer finds any missing mutes).