Optimizing refactoring: convert EnsureNotLocked() from extension to Method

[SergeiPavlov]

I see EnsureNotLocked() is a hotspot during profiling.
All invocations of EnsureNotLocked() are for this. We can convert it from extension to method.
This adds following optimizations:

• No need for null check because C# guarantees this is non-null (it can be avoided by manual IL code generating).
• Access to IsLocked propery via interface is much slower than access to it as a field.

Also:

• Fix documentation description of ILockable.Lock() - the only its implementation calls Lock(recursive: true). It is recursive by default.
• Remove Unlock() method as non-usecd