TS-2519: Stop using the ambiguous RECP_NULL

RECP_NULL is ambigious. Should stats with this prsistence type be
saved or not?

The current behavior is that all stats that are not RECP_NON_PERSISTENT
are persisted. Change all registrations of RECP_NULL stats to use
RECP_PERSISTENT instead. Change the build version stats to
RECP_NON_PERSISTENT, since we want these to update when Traffic
Server is upgraded.
TS-2519: make using RECP_NULL a compilation error

RECP_NULL can still be useful to represent the case when a record
does not have a persistence type. It might be a configuration record,
for example. However, stats records shold never be registered as
RECP_NULL, because that's ambiguous as to whether it should be
persisted This change add some template helpers to ensure that any
attempt to register a RECP_NULL stat dies in a horrible shower of
compiler errors.
TS-2519: handle stat persistence type changes

When Traffic Server is upgraded, it is possible for stats to change
their persistence type. If it changes from persistent to non-persistent,
we don't want to preserve the previous value. Detect this case when
we load the records snapshot and also when we register stats.
TS-2519: update CHANGES