notary: fix possible deadlock in UpdateNotaryNodes
`UpdateNotaryNodes` takes account then request mutex, and `PostPersist` takes
them in a different order. Because they are executed concurrently a deadlock
can appear.
```
2021-07-23T11:06:58.3732405Z panic: test timed out after 10m0s
2021-07-23T11:06:58.3732642Z
2021-07-23T11:06:58.3742610Z goroutine 7351 [semacquire, 9 minutes]:
2021-07-23T11:06:58.3743140Z sync.runtime_SemacquireMutex(0xc00010e4dc, 0x1100000000, 0x1)
2021-07-23T11:06:58.3743747Z /opt/hostedtoolcache/go/1.14.15/x64/src/runtime/sema.go:71 +0x47
2021-07-23T11:06:58.3744222Z sync.(*Mutex).lockSlow(0xc00010e4d8)
2021-07-23T11:06:58.3744742Z /opt/hostedtoolcache/go/1.14.15/x64/src/sync/mutex.go:138 +0x1c1
2021-07-23T11:06:58.3745209Z sync.(*Mutex).Lock(0xc00010e4d8)
2021-07-23T11:06:58.3745692Z /opt/hostedtoolcache/go/1.14.15/x64/src/sync/mutex.go:81 +0x7d
2021-07-23T11:06:58.3746162Z sync.(*RWMutex).Lock(0xc00010e4d8)
2021-07-23T11:06:58.3746764Z /opt/hostedtoolcache/go/1.14.15/x64/src/sync/rwmutex.go:98 +0x4a
2021-07-23T11:06:58.3747699Z github.com/nspcc-dev/neo-go/pkg/services/notary.(*Notary).UpdateNotaryNodes(0xc00010e480, 0xc000105b90, 0x1, 0x1)
2021-07-23T11:06:58.3748621Z /home/runner/work/neo-go/neo-go/pkg/services/notary/node.go:44 +0x3ba
2021-07-23T11:06:58.3749367Z github.com/nspcc-dev/neo-go/pkg/core.TestNotary(0xc0003677a0)
2021-07-23T11:06:58.3750116Z /home/runner/work/neo-go/neo-go/pkg/core/notary_test.go:594 +0x2dba
2021-07-23T11:06:58.3750641Z testing.tRunner(0xc0003677a0, 0x16f3308)
2021-07-23T11:06:58.3751202Z /opt/hostedtoolcache/go/1.14.15/x64/src/testing/testing.go:1050 +0x1ec
2021-07-23T11:06:58.3751696Z created by testing.(*T).Run
2021-07-23T11:06:58.3752225Z /opt/hostedtoolcache/go/1.14.15/x64/src/testing/testing.go:1095 +0x538
2021-07-23T11:06:58.3752573Z
2021-07-23T11:06:58.3771319Z goroutine 7340 [semacquire, 9 minutes]:
2021-07-23T11:06:58.3772048Z sync.runtime_SemacquireMutex(0xc00010e504, 0x0, 0x0)
2021-07-23T11:06:58.3772889Z /opt/hostedtoolcache/go/1.14.15/x64/src/runtime/sema.go:71 +0x47
2021-07-23T11:06:58.3773581Z sync.(*RWMutex).RLock(0xc00010e4f8)
2021-07-23T11:06:58.3774310Z /opt/hostedtoolcache/go/1.14.15/x64/src/sync/rwmutex.go:50 +0xa4
2021-07-23T11:06:58.3775449Z github.com/nspcc-dev/neo-go/pkg/services/notary.(*Notary).getAccount(0xc00010e480, 0x0)
2021-07-23T11:06:58.3776626Z /home/runner/work/neo-go/neo-go/pkg/services/notary/node.go:51 +0x51
2021-07-23T11:06:58.3778270Z github.com/nspcc-dev/neo-go/pkg/services/notary.(*Notary).finalize(0xc00010e480, 0xc0003b2630, 0xa97df1bc78dd5787, 0xcc8a4d69e7f5d62a, 0x1a4d7981bd86b087, 0xbafdb720c93480b3, 0x0, 0x0)
2021-07-23T11:06:58.3779845Z /home/runner/work/neo-go/neo-go/pkg/services/notary/notary.go:306 +0x54
2021-07-23T11:06:58.3781022Z github.com/nspcc-dev/neo-go/pkg/services/notary.(*Notary).PostPersist(0xc00010e480)
2021-07-23T11:06:58.3782232Z /home/runner/work/neo-go/neo-go/pkg/services/notary/notary.go:297 +0x662
2021-07-23T11:06:58.3782989Z github.com/nspcc-dev/neo-go/pkg/services/notary.(*Notary).Run(0xc00010e480)
2021-07-23T11:06:58.3783941Z /home/runner/work/neo-go/neo-go/pkg/services/notary/notary.go:148 +0x3cb
2021-07-23T11:06:58.3784702Z created by github.com/nspcc-dev/neo-go/pkg/core.TestNotary
2021-07-23T11:06:58.3785451Z /home/runner/work/neo-go/neo-go/pkg/core/notary_test.go:132 +0x6e0
```
Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>
e2f2addf95