Use Last[Read/Write]Count() in socket stream operations

Using LastRead() was MT-unsafe when the same socket was used for both
writing and reading from different threads.

It's not clear if this change is sufficient to make wxSocket fully
MT-safe in this scenario, but it does help and there should be no
negative effects from doing this.

Closes #17787.

Closes https://github.com/wxWidgets/wxWidgets/pull/761

docs/changes.txt

@@ -70,6 +70,7 @@ Changes in behaviour which may result in build errors
 All:
 
 - Make wxList and wxVector iterators conform to input iterator requirements.
+- Fix MT-safety problem when reading and writing from wxSocket (jkubalik).
 
 wxGTK: