RFR: 8214278: Cleanup process_completed_threshold and related state

[Kim Barrett]

This change addresses two CRs:
8214278: Cleanup process_completed_threshold and related state  
8156696: Simplify PtrQueueSet initialization 

This change is based on the change for 8214202:
DirtyCardQueueSet::get_completed_buffer should not clear
_process_completed, which is already out for review.

Please review this change to some of the data members in PtrQueueSet,
to simplify their semantics and usage.  The affected data members are

_process_completed_threshold
_max_completed_queue 
_completed_queue_padding

The types of all of them are now size_t (some were int).  There is no
longer any special treatment of zero (and negative is no longer
possible).  The only special treatment now required is the possibility
of overflow when adding max completed and padding.

The members are now initialized to extreme values (either 0 or max, as
appropriate), with PtrQueueSet::initialize no longer taking any
associated arguments.  Instead, through proper choice of the initial
values, some require no further initialization, and the rest are
updated via the associated setters.  (This addresses JDK-8156696.)

Added missing const qualifiers for associated reader functions.

Finally, the following renamings were made, for improved naming consistency:

process_completed_threshold => process_completed_buffers_threshold
max_completed_queue => max_completed_buffers
completed_queue_padding => completed_buffers_padding

(There are still some naming inconsistencies around "completed buffers",
which I'm planning to address in future RFEs.)

CR:
https://bugs.openjdk.java.net/browse/JDK-8214278
https://bugs.openjdk.java.net/browse/JDK-8156696

Webrev:
http://cr.openjdk.java.net/~kbarrett/8214278/open.00/

Testing:
mach5 tier1-5