core: protect (*Blockchain).keyCache with a lock
Avoid failures like this:
fatal error: concurrent map writes
goroutine 103 [running]:
runtime.throw(0xca6fe3, 0x15)
/usr/lib64/go/1.12/src/runtime/panic.go:617 +0x72 fp=0xc000687988 sp=0xc000687958 pc=0x42d8d2
runtime.mapassign(0xb9d4a0, 0xc0000b1f80, 0xc000687a6c, 0x13bd940)
/usr/lib64/go/1.12/src/runtime/map.go:590 +0x5e3 fp=0xc000687a10 sp=0xc000687988 pc=0x40e2a3
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHashAgainstScript(0xc0000f8000, 0x1300d1f5a3d348be, 0x78607089e4fe9fab, 0xc0a21e4f71, 0xc000d16070, 0x1e2b8881f8178e92, 0xd589cfa965287bb8, 0x24a4e4f292ebc797, 0xb90d3120d878e619, 0xc0001d01e0, ...)
/home/rik/dev/neo-go/pkg/core/blockchain.go:1866 +0x371 fp=0xc000687ad8 sp=0xc000687a10 pc=0x984ed1
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHeaderWitnesses(0xc0000f8000, 0xc000d16000, 0xc00042d950, 0x1, 0xc00008a6c0)
/home/rik/dev/neo-go/pkg/core/blockchain.go:1914 +0x209 fp=0xc000687c90 sp=0xc000687ad8 pc=0x985999
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).verifyHeader(0xc0000f8000, 0xc000d16000, 0xc00042d950, 0xf0ee0049d611101f, 0xf8967398a8f9351b)
/home/rik/dev/neo-go/pkg/core/blockchain.go:1254 +0x235 fp=0xc000687d70 sp=0xc000687c90 pc=0x97e4e5
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).addHeaders(0xc0000f8000, 0x1, 0xc000d12000, 0x7d0, 0x7d0, 0x0, 0x1)
/home/rik/dev/neo-go/pkg/core/blockchain.go:369 +0x266 fp=0xc000687e68 sp=0xc000687d70 pc=0x973f46
github.com/nspcc-dev/neo-go/pkg/core.(*Blockchain).AddHeaders(0xc0000f8000, 0xc000d12000, 0x7d0, 0x7d0, 0x0, 0xc0003640c0)
/home/rik/dev/neo-go/pkg/core/blockchain.go:338 +0x51 fp=0xc000687eb0 sp=0xc000687e68 pc=0x973cb1
github.com/nspcc-dev/neo-go/pkg/network.(*Server).handleHeadersCmd(0xc00022c140, 0xdd0ca0, 0xc000374000, 0xc00000c020)
/home/rik/dev/neo-go/pkg/network/server.go:431 +0x74 fp=0xc000687fc0 sp=0xc000687eb0 pc=0x9ce5e4
runtime.goexit()
/usr/lib64/go/1.12/src/runtime/asm_amd64.s:1337 +0x1 fp=0xc000687fc8 sp=0xc000687fc0 pc=0x45ca51
created by github.com/nspcc-dev/neo-go/pkg/network.(*Server).handleMessage
/home/rik/dev/neo-go/pkg/network/server.go:705 +0xa4a
...
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