Sentinel: Add initial quorum bounds check

Without this commit, Redis can generate an invalid config:

```haskell
127.0.0.1:26379> sentinel monitor bob-master 127.0.0.1 6379 0
```

Sentinel accepts that command, but then after a restart, we get...

```
*** FATAL CONFIG FILE ERROR ***
Reading the configuration file, at line 4
>>> 'sentinel monitor bob-master 127.0.0.1 6379 0'
Quorum must be 1 or greater.
```