services: fix Oracle responces mutex
Solves the following problem:
=== RUN TestOracleFull
logger.go:130: 2021-02-04T09:25:16.305Z INFO P2PNotaryRequestPayloadPool size is not set or wrong, setting default value {"P2PNotaryRequestPayloadPoolSize": 1000}
logger.go:130: 2021-02-04T09:25:16.306Z INFO no storage version found! creating genesis block
logger.go:130: 2021-02-04T09:25:27.687Z DEBUG done processing headers {"headerIndex": 1, "blockHeight": 0, "took": "2.413398ms"}
logger.go:130: 2021-02-04T09:25:27.696Z DEBUG done processing headers {"headerIndex": 2, "blockHeight": 1, "took": "1.138196ms"}
logger.go:130: 2021-02-04T09:25:28.680Z INFO blockchain persist completed {"persistedBlocks": 2, "persistedKeys": 173, "headerHeight": 2, "blockHeight": 2, "took": "166.793µs"}
fatal error: sync: Unlock of unlocked RWMutex
goroutine 6157 [running]:
runtime.throw(0x115dfdb, 0x20)
/usr/local/go/src/runtime/panic.go:1116 +0x72 fp=0xc000297ca0 sp=0xc000297c70 pc=0x44f432
sync.throw(0x115dfdb, 0x20)
/usr/local/go/src/runtime/panic.go:1102 +0x35 fp=0xc000297cc0 sp=0xc000297ca0 pc=0x44f3b5
sync.(*RWMutex).Unlock(0xc000135300)
/usr/local/go/src/sync/rwmutex.go:129 +0xf3 fp=0xc000297d00 sp=0xc000297cc0 pc=0x4a1ac3
github.com/nspcc-dev/neo-go/pkg/services/oracle.(*Oracle).Run(0xc000135180)
/go/src/github.com/nspcc-dev/neo-go/pkg/services/oracle/oracle.go:189 +0x82b fp=0xc000297fd8 sp=0xc000297d00 pc=0xe13b0b
runtime.goexit()
/usr/local/go/src/runtime/asm_amd64.s:1373 +0x1 fp=0xc000297fe0 sp=0xc000297fd8 pc=0x4834d1
created by github.com/nspcc-dev/neo-go/pkg/core.TestOracleFull
/go/src/github.com/nspcc-dev/neo-go/pkg/core/oracle_test.go:276 +0x3f1
b0563ad0d7