storage: provide a way to escape from SeekAsync goroutine
A routine blocked on channel send here can't really exit, so avoid goroutine
leak:
goroutine 2813725 [chan send, 6 minutes]:
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync.func1.1(0xc01a7118f7, 0x2, 0x25, 0xc01a7118f9, 0x23, 0x23, 0xc0366c7c01)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:120 +0x86
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek.func2(0xc0079e7920, 0xa, 0x30, 0xc0079e792a, 0x26, 0x26, 0x1)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:183 +0x347
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek(0xc000458480, 0x135c028, 0xc0000445d0, 0xc00f1721d0, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:224 +0x4f4
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).Seek(0xc000458480, 0xc00f1721d0, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, 0xc0357c6620)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:110 +0x8a
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek(0xc0331a4f00, 0x135bff0, 0xc00ae26ec0, 0xc00f1721d0, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:210 +0x379
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync.func1(0xc0331a4f00, 0x135bff0, 0xc00ae26ec0, 0xc00f1721d0, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:119 +0xc5
created by github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:118 +0xc8
goroutine 2822823 [chan send, 6 minutes]:
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync.func1.1(0xc011859b77, 0x3, 0x3, 0xc017bea8d0, 0x26, 0x26, 0xc00f1afc00)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:120 +0x86
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek.func2(0xc011859b60, 0xa, 0xa, 0xc017bea8a0, 0x26, 0x26, 0xc00ad9fb00)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:200 +0x47e
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek.func2(0xc01d5d8c90, 0xa, 0x30, 0xc01d5d8c9a, 0x26, 0x26, 0x1)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:200 +0x47e
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek(0xc035e12900, 0x135c028, 0xc0000445d0, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:224 +0x4f4
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).Seek(0xc035e12900, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, 0xc030c9e0e0)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:110 +0x8a
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek(0xc000458480, 0x135c028, 0xc0000445d0, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:210 +0x379
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).Seek(0xc000458480, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, 0xc030c9e070)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:110 +0x8a
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).seek(0xc00b340c60, 0x135bff0, 0xc00f1afbc0, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:210 +0x379
github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync.func1(0xc00b340c60, 0x135bff0, 0xc00f1afbc0, 0xc01773bf60, 0x7, 0x7, 0x0, 0x0, 0x0, 0x0, ...)
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:119 +0xc5
created by github.com/nspcc-dev/neo-go/pkg/core/storage.(*MemCachedStore).SeekAsync
github.com/nspcc-dev/neo-go/pkg/core/storage/memcached_store.go:118 +0xc8
...