Remove or replace old-style casts that cast away constness

Ono of the impediments to thread safety is casting away constness.  Old style casts that do this are especially bad because they are harder to find than const_cast.
I ran the clang static analyzer on all the framework code minus the unit tests.  All occurrences of old-style casts that cast away constness were in a single package, IOPool/Streamer.
This pull request makes very simple changes to remove or replace those casts.  In most cases, casting away constness was not needed, and the cast was removed or modified so that it no longer casts away constness.
In the places where it was not trivial to remove the casting away constness, a const_cast is now used to cast away constness, so no old style cast still does.
In a very few cases, some old style casts that did not remove constness were removed or replaced by C++ casts.