According to a CPU profile acquired while running TPC-E, the read lock acquired
in `Replica.IsInitialized` is the second most expensive read lock in terms of
CPU cycles consumed. This doesn't mean that it's the most expensive in terms of
blocking, but it still indicates that the lock is frequently acquired.

```
----------------------------------------------------------+-------------
                                             0.29s 43.28% |   github.com/cockroachdb/pebble/internal/cache.(*shard).Get github.com/cockroachdb/pebble/internal/cache/external/com_github_cockroachdb_pebble/internal/cache/clockpro.go:117 (inline)
                                             0.05s  7.46% |   github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Replica).IsInitialized github.com/cockroachdb/cockroach/pkg/kv/kvserver/pkg/kv/kvserver/replica_init.go:249 (inline)
...
     0.67s  0.39% 24.00%      0.67s  0.39%                | sync.(*RWMutex).RLock GOROOT/src/sync/rwmutex.go:61
----------------------------------------------------------+-------------
```

The two main callers of this method are `Store.Send` (on each request) and
`Replica.handleRaftReadyRaftMuLocked` (on each raft ready iteration):

```
----------------------------------------------------------+-------------
                                             0.04s 80.00% |   github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Store).Send github.com/cockroachdb/cockroach/pkg/kv/kvserver/pkg/kv/kvserver/store_send.go:176
                                             0.01s 20.00% |   github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Replica).handleRaftReadyRaftMuLocked github.com/cockroachdb/cockroach/pkg/kv/kvserver/pkg/kv/kvserver/replica_raft.go:829
     0.04s 0.023% 0.023%      0.05s 0.029%                | github.com/cockroachdb/cockroach/pkg/kv/kvserver.(*Replica).IsInitialized github.com/cockroachdb/cockroach/pkg/kv/kvserver/pkg/kv/kvserver/replica_init.go:251
                                             0.01s 20.00% |   sync.(*RWMutex).RUnlock GOROOT/src/sync/rwmutex.go:81
----------------------------------------------------------+-------------
```

This commit makes the method cheaper by replacing the lock with an atomic
read. Once a replica is initialized, it stays initialized, so this state
is well suited to be stored in an atomic value.