RFR: 8228692: BitMap::reallocate might not clear some bits

[Kim Barrett]

Please review this change to BitMap reallocation that ensures all
newly added bits are cleared when requested.  Previously, there were
circumstances where some might not be.  Includes a new gtest for the
specific issue.

This change also includes a fix for JDK-8235714.  Contrary to previous
comments and bug updates, it was not fixed by JDK-8241581 after all.
This was noticed while testing the change for JDK-8228692.

CR:
https://bugs.openjdk.java.net/browse/JDK-8228692
https://bugs.openjdk.java.net/browse/JDK-8235714

Webrev:
https://cr.openjdk.java.net/~kbarrett/8228692/open.00/

Testing:
mach5 tier1

Local (linux-x64) BitMap tests, including the new test.  The new test
(1) failed before the changes, (2) still failed after either one but
not both of the fix for relocate and the fix for count_one_bits, (3)
and passes with both fixes.