Change API/ABI and withdraw 4.2.0

4.2.0 was binary incompatible in spite of there being no deletions or
changes to any public methods.  As such, we have to bump the ABI and
are fixing some API breakage while we're at it.

Previous 4.3.0 target is now 5.1.0.
Update doc for 5.0.0
Update valgrid suppressions
Prepare 5.0.0 release