ClientConn needs to be protected by mutex

[scudette]

Race detector complains:

Write at 0x00c0000b63c0 by goroutine 104:
  github.com/processout/grpc-go-pool.(*ClientConn).Close()
      /home/mic/go/pkg/mod/github.com/processout/grpc-go-pool@v1.2.2-0.20200228131710-c0fcf3af0014/pool.go:236 +0x344
  github.com/processout/grpc-go-pool.(*ClientConn).Close-fm()
      /home/mic/go/pkg/mod/github.com/processout/grpc-go-pool@v1.2.2-0.20200228131710-c0fcf3af0014/pool.go:195 +0x39

Previous read at 0x00c0000b63c0 by goroutine 101:
  github.com/processout/grpc-go-pool.(*ClientConn).Close()
      /home/mic/go/pkg/mod/github.com/processout/grpc-go-pool@v1.2.2-0.20200228131710-c0fcf3af0014/pool.go:220 +0x204
  github.com/processout/grpc-go-pool.(*ClientConn).Close-fm()
      /home/mic/go/pkg/mod/github.com/processout/grpc-go-pool@v1.2.2-0.20200228131710-c0fcf3af0014/pool.go:195 +0x39

[dzvancuks]

I experience same issue with client racing.

1. Call pool Get to grab connection
2. Close pool in new go routine
3. Call connection Close

WARNING: DATA RACE
Write at 0x00c0000248b0 by goroutine 8:
  github.com/processout/grpc-go-pool.(*Pool).Close()
      github.com/processout/grpc-go-pool/pool.go:111 +0xb3

Previous read at 0x00c0000248b0 by goroutine 7:
  github.com/processout/grpc-go-pool.(*ClientConn).Close()
      github.com/processout/grpc-go-pool/pool.go:217 +0x324

But this is about direct c.pool.clients access from within client. This one should also be refactored