store/cache: speed up + conserve memory by using map clearing idiom in Reset (#6700)

Noticed during my audit, the code for cache.CommitKVStoreCacheManager.Reset()
discarded the prior on every invocation i.e.

    m = make(map[T(key)]T(value))

However, this can be made fast and conserve memory by using the map clearing idiom
that the Go compiler recognizes i.e.

    for key := range m {
        delete(m, key)
    }

and turns into very fast code, instead of the extraneous map discarding.

The speed up generated is:

```shell
$ benchstat before after
name     old time/op    new time/op    delta
Reset-8     204ns ± 2%      66ns ± 9%   -67.86%  (p=0.000 n=20+20)

name     old alloc/op   new alloc/op   delta
Reset-8      384B ± 0%        0B       -100.00%  (p=0.000 n=20+20)

name     old allocs/op  new allocs/op  delta
Reset-8      3.00 ± 0%      0.00       -100.00%  (p=0.000 n=20+20)
```

Fixes #6681

store/cache/benchmark_test.go

@@ -0,0 +1,48 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/cosmos/cosmos-sdk/store/types"
+)
+
+func freshMgr() *CommitKVStoreCacheManager {
+	return &CommitKVStoreCacheManager{
+		caches: map[string]types.CommitKVStore{
+			"a1":           nil,
+			"alalalalalal": nil,
+		},
+	}
+}
+
+func populate(mgr *CommitKVStoreCacheManager) {
+	mgr.caches["this one"] = (types.CommitKVStore)(nil)
+	mgr.caches["those ones are the ones"] = (types.CommitKVStore)(nil)
+	mgr.caches["very huge key right here and there are we going to ones are the ones"] = (types.CommitKVStore)(nil)
+}
+
+func BenchmarkReset(b *testing.B) {
+	mgr := freshMgr()
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		mgr.Reset()
+		if len(mgr.caches) != 0 {
+			b.Fatal("Reset failed")
+		}
+		populate(mgr)
+		if len(mgr.caches) == 0 {
+			b.Fatal("populate failed")
+		}
+		mgr.Reset()
+		if len(mgr.caches) != 0 {
+			b.Fatal("Reset failed")
+		}
+	}
+
+	if mgr == nil {
+		b.Fatal("Impossible condition")
+	}
+}

store/cache/cache.go

@@ -81,7 +81,12 @@ func (cmgr *CommitKVStoreCacheManager) Unwrap(key types.StoreKey) types.CommitKV
 
 // Reset resets in the internal caches.
 func (cmgr *CommitKVStoreCacheManager) Reset() {
-	cmgr.caches = make(map[string]types.CommitKVStore)
+	// Clear the map.
+	// Please note that we are purposefully using the map clearing idiom.
+	// See https://github.com/cosmos/cosmos-sdk/issues/6681.
+	for key := range cmgr.caches {
+		delete(cmgr.caches, key)
+	}
 }
 
 // CacheWrap returns the inter-block cache as a cache-wrapped CommitKVStore.