[#48] pool: add `Close` method
Fix occasional panic in tests:
```
> for i in (seq 1 100); go test -race -count=1 ./pool/... ; end
...
{"level":"warn","ts":1635251466.567485,"caller":"pool/pool.go:122","msg":"failed to create neofs session token for client","address":"peer0","error":"error session"}
panic: Fail in goroutine after TestTwoNodes has completed
goroutine 6 [running]:
testing.(*common).Fail(0xc0002e1380)
/usr/lib/go/src/testing/testing.go:710 +0x1b4
testing.(*common).FailNow(0xc0002e1380)
/usr/lib/go/src/testing/testing.go:732 +0x2f
testing.(*common).Fatalf(0xc000074070, {0xd9d816, 0x2e}, {0xc000094050, 0x5, 0x5})
/usr/lib/go/src/testing/testing.go:830 +0x85
github.com/golang/mock/gomock.(*Controller).Call.func1(0xc0002f4120, {0xd68380, 0xc0002dac30}, {0xd8847f, 0xc}, {0xc000074020, 0x1, 0x1})
/home/dzeta/go/pkg/mod/github.com/golang/mock@v1.6.0/gomock/controller.go:231 +0x44d
github.com/golang/mock/gomock.(*Controller).Call(0xc0002f4120, {0xd68380, 0xc0002dac30}, {0xd8847f, 0xc}, {0xc000074020, 0x1, 0x1})
/home/dzeta/go/pkg/mod/github.com/golang/mock@v1.6.0/gomock/controller.go:247 +0xce
github.com/nspcc-dev/neofs-sdk-go/pool.(*MockClient).EndpointInfo(0xc0002dac30, {0xe85528, 0xc00008a120}, {0x0, 0x0, 0x0})
/home/dzeta/repo/neofs-sdk-go/pool/mock_test.go:186 +0x298
github.com/nspcc-dev/neofs-sdk-go/pool.updateNodesHealth.func1(0x1, {0xe950d8, 0xc0002dac30})
/home/dzeta/repo/neofs-sdk-go/pool/pool.go:183 +0x188
created by github.com/nspcc-dev/neofs-sdk-go/pool.updateNodesHealth
/home/dzeta/repo/neofs-sdk-go/pool/pool.go:174 +0x233
```
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>
cd5e08d725