more baseline cleanups from Async Monitor Deflation project

This sub-task captures updates to the baseline code that came up
during the Async Monitor Deflation project design review and code
review email threads.
 
The cleanups include:

- rename ObjectMonitor::_count -> ObjectMonitor::_contentions
- update comment in objectMonitor.hpp for _contentions field declaration
- delete unused count_offset_in_bytes()
- delete count() getter
- replace uses of 'count()' with 'contentions()'
- deleted declared but undefined set_count()
- delete some stale TODO-FIXME comments
- update comments in ObjectSynchronizer::FastHashCode()
- change 'hashcode' -> 'hash code' for consistency; the consistent
  forms are: 'hash code', 'Hash Code', 'hashCode' and 'HashCode'
- add more assert() diagnostics for incoming object header in inflate()
- add more assert() diagnostics for enter()'s TrySpin() branch
- add more assert() diagnostics in ObjectMonitor::clear()
- add more assert() and guarantee() diagnostics in
  ObjectSynchronizer::deflate_monitor()
- get rid of unnecessary '(address)' casts

One of the changes is in print output:

- st->print("{count=0x%08x,waiters=0x%08x"
+ st->print("{contentions=0x%08x,waiters=0x%08x"

I searched for tests that depended on '{count=.*,waiters=' and did not find any.