*: use clear() to clear maps

Supposedly more efficient since we can avoid some memory management dances.

Memory pool agrees:

goos: linux
goarch: amd64
pkg: github.com/nspcc-dev/neo-go/pkg/core/mempool
cpu: AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics
                       │  pool.old   │              pool.new               │
                       │   sec/op    │   sec/op     vs base                │
Pool/one,_incr_fee-16    12.44m ± 1%   12.51m ± 1%   +0.55% (p=0.029 n=10)
Pool/many,_same_fee-16   4.960m ± 2%   3.100m ± 1%  -37.50% (p=0.000 n=10)
Pool/many,_incr_fee-16   16.03m ± 2%   14.11m ± 1%  -12.00% (p=0.000 n=10)
Pool/one,_same_fee-16                  1.742m ± 1%
geomean                  9.964m        5.556m       -17.92%

                       │    pool.old     │                pool.new                │
                       │      B/op       │      B/op       vs base                │
Pool/one,_incr_fee-16     8.117Ki ± 120%   7.101Ki ± 128%  -12.52% (p=0.022 n=10)
Pool/many,_same_fee-16   3941.2Ki ±   0%   805.4Ki ±   0%  -79.56% (p=0.000 n=10)
Pool/many,_incr_fee-16   3936.2Ki ±   0%   829.8Ki ±   0%  -78.92% (p=0.000 n=10)
Pool/one,_same_fee-16                      12.98Ki ±  10%
geomean                   501.2Ki          88.59Ki         -66.47%

                       │   pool.old   │               pool.new               │
                       │  allocs/op   │  allocs/op    vs base                │
Pool/one,_incr_fee-16     28.00 ± 21%    24.00 ± 21%  -14.29% (p=0.002 n=10)
Pool/many,_same_fee-16   40.38k ±  0%   40.03k ±  0%   -0.86% (p=0.000 n=10)
Pool/many,_incr_fee-16   40.38k ±  0%   40.04k ±  0%   -0.85% (p=0.000 n=10)
Pool/one,_same_fee-16                    23.00 ±  4%
geomean                  3.574k          969.8         -5.55%

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/core/mempool/mem_pool.go

@@ -351,8 +351,8 @@ func (mp *Pool) RemoveStale(isOK func(*transaction.Transaction) bool, feer Feer)
 	// We can reuse already allocated slice
 	// because items are iterated one-by-one in increasing order.
 	newVerifiedTxes := mp.verifiedTxes[:0]
-	mp.fees = make(map[util.Uint160]utilityBalanceAndFees) // it'd be nice to reuse existing map, but we can't easily clear it
-	mp.conflicts = make(map[util.Uint256][]util.Uint256)
+	clear(mp.fees)
+	clear(mp.conflicts)
 	height := feer.BlockHeight()
 	var (
 		staleItems []item

pkg/core/mpt/trie.go

@@ -543,7 +543,7 @@ func (t *Trie) Collapse(depth int) {
 		panic("negative depth")
 	}
 	t.root = collapse(depth, t.root)
-	t.refcount = make(map[util.Uint256]*cachedNode)
+	clear(t.refcount)
 }
 
 func collapse(depth int, node Node) Node {

pkg/vm/stackitem/serialization.go

@@ -133,9 +133,7 @@ func (w *SerializationContext) Serialize(item Item, protected bool) ([]byte, err
 	if w.data != nil {
 		w.data = w.data[:0]
 	}
-	for k := range w.seen {
-		delete(w.seen, k)
-	}
+	clear(w.seen)
 	err := w.serialize(item)
 	if err != nil && protected {
 		if w.data == nil {