Data race when calling SetConsistency

Please answer these questions before submitting your issue. Thanks!

### What version of Cassandra are you using?

Docker image cassandra:3.11.

### What version of Gocql are you using?

```
$ grep gocql go.mod
github.com/gocql/gocql v0.0.0-20200624222514-34081eda590e
```

### What did you do?

```
sess, err := cluster.CreateSession()
if err != nil {
return nil, err
}
sess.SetConsistency(consistency)
```

Elsewhere:
```
sess.Query(...)
```

### What did you expect to see?

No data race when running with `-race`.

### What did you see instead?

```
=== FAIL: physical/cassandra TestCassandraBackend (25.07s)
==================
WARNING: DATA RACE
Read at 0x00c0001d8380 by goroutine 25:
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*Conn).UseKeyspace()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/conn.go:1235 +0x186
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*hostConnPool).connect()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/connectionpool.go:534 +0x53d
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*hostConnPool).connectMany.func1()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/connectionpool.go:487 +0x79

Previous write at 0x00c0001d8380 by goroutine 7:
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*Session).SetConsistency()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/session.go:336 +0x54
github.com/hashicorp/vault/physical/cassandra.NewCassandraBackend()
/go/src/github.com/hashicorp/vault/physical/cassandra/cassandra.go:144 +0x917
github.com/hashicorp/vault/physical/cassandra.TestCassandraBackend()
/go/src/github.com/hashicorp/vault/physical/cassandra/cassandra_test.go:29 +0x2dc
testing.tRunner()
/usr/local/go/src/testing/testing.go:1039 +0x1eb

Goroutine 25 (running) created at:
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*hostConnPool).connectMany()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/connectionpool.go:485 +0x159
github.com/hashicorp/vault/vendor/github.com/gocql/gocql.(*hostConnPool).fill.func1()
/go/src/github.com/hashicorp/vault/vendor/github.com/gocql/gocql/connectionpool.go:439 +0x42

Goroutine 7 (running) created at:
testing.(*T).Run()
/usr/local/go/src/testing/testing.go:1090 +0x700
testing.runTests.func1()
/usr/local/go/src/testing/testing.go:1334 +0xa6
testing.tRunner()
/usr/local/go/src/testing/testing.go:1039 +0x1eb
testing.runTests()
/usr/local/go/src/testing/testing.go:1332 +0x527
testing.(*M).Run()
/usr/local/go/src/testing/testing.go:1249 +0x43f
main.main()
_testmain.go:46 +0x223
```

---

To reproduce, clone `github.com/hashicorp/vault`, then:

```
go test -race ./physical/cassandra
```

The issue looks to be that although `Session.SetConsistency` uses a lock to protect writes to `s.cons`, `Conn.UseKeyscape` reads `c.session.cons` without any mechanism to manage thread safety.

Run `go test -race ./physical/cassandra` to reproduce the report. Read `session.go` at `Session.SetConsistency`, then follow the stack trace through `conn.go` and `connectionpool.go`; done means the reported race no longer appears under the race detector.

Written by the indexing model from the issue text.