Analyzer + codefix: Mark objects used for locking as readonly (new or RCS1169 derivative)

Consider:

```
object locker = new object();

void LockedAction()
{
lock (locker)
{
// ... do something requiring locking...
}
}
```

But any code can reassign the value of `locker`:

```
void SomethingElse()
{
locker = new object();
}
```

The end result is that `LockedAction` is no longer thread-safe and Bad Things™ will happen when it is invoked concurrently.

To prevent this bug, `locker` should be declared as `readonly` so that it can never be reassigned:

```
readonly object locker = new object();

void SomethingElse()
{
locker = new object(); // compile error
}
```

I'm aware this is already covered by [RSC1169](https://github.com/JosefPihrt/Roslynator/blob/master/docs/analyzers/RCS1169.md), but this is a special and IMO more important case.

Start by reading docs/analyzers/RCS1169.md, which the issue identifies as related work, and compare its scope with the locking-specific behavior described here. Determine how the analyzer and codefix should recognize objects used in lock statements and make the declaration readonly; done means the unsafe reassignment is diagnosed or prevented by the proposed fix.

Written by the indexing model from the issue text.