Replicate readonly scripts with SCRIPT LOAD.

There is an inconsistency in the way read-only scripts are replicated:
- On EVAL, a script won't be replicated if it's read-only.
- On EVALSHA it will always be replicated unless it's already in the replication cache (i.e. known to be replicated).

In the following sequence, the EVALSHA on the slave fails:
```
master> EVAL <some readonly script>
slave> EVALSHA <sha1 of script>
```

In the following sequence, it will succeed:
```
master> EVAL <some readonly script>
master> EVALSHA <sha1 of script>
slave> EVALSHA <sha1 of script>
```

The proposed solution is to simply use SCRIPT LOAD on read-only EVAL if the script was not previously cached.