Fixed deadlock in System.Resources.ResourceSet

Fixed deadlock in System.Resources.ResourceSet

ResourceSet internal enumerator uses the following lock order:
1. lock _resCache (as _reader._resCache)
       in System.Resources.ResourceSet+ResourceEnumerator.Entry
2. lock reader (this)
       in the body on of _reader.AllocateStringForNameIndex

RuntimeResourceSet.GetObject uses the following lock order:
1. Lock on reader
2. Lock on _resCache
    (on the same instance as passed to the reader ctor)

That causes deadlock when one thread tries to enumerate the ResourceSet
while another thread reads a specific object from the same set.

Added lock in System.Resources.ResourceSet+ResourceEnumerator.Entry,
so both code paths locks on reader first.

Fix Connect Issue ID #580259