Fix SNMP cacheNumObjCount -- number of cached objects (#2053)

Fix SNMP cacheNumObjCount -- number of cached objects (#2053)

SNMP counter cacheNumObjCount used StoreEntry::inUseCount() stats. For
Squid instances using a rock cache_dirs or a shared memory cache, the
number of StoreEntry objects in use is usually very different from the
number of cached objects because these caches do not use StoreEntry
objects as a part of their index. For all instances, inUseCount() also
includes ongoing transactions and internal tasks that are not related to
cached objects at all.

We now use the sum of the counters already reported on "on-disk objects"
and "Hot Object Cache Items" lines in "Internal Data Structures" section
of `mgr:info` cache manager report. Due to floating-point arithmetic,
these stats are approximate, but it is best to keep SNMP and cache
manager reports consistent.

This change does not fix SNMP Gauge32 overflow bug: Caches with 2^32 or
more objects continue to report wrong/smaller cacheNumObjCount values.

### On MemStore::getStats() and StoreInfoStats changes

To include the number of memory-cached objects while supporting SMP
configurations with shared memory caches, we had to change how cache
manager code aggregates StoreInfoStats::mem data collected from SMP
worker processes. Before these changes, `StoreInfoStats::operator +=()`
used a mem.shared data member to trigger special aggregation code hack,
but

* SNMP-specific code cannot benefit from that StoreInfoStats aggregation
  because SNMP code exchanges simple counters rather than StoreInfoStats
  objects. `StoreInfoStats::operator +=()` is never called by SNMP code.
  Instead, SNMP uses Snmp::Pdu::aggregate() and friends.

* We could not accommodate SNMP by simply adding special aggregation
  hacks directly to MemStore::getStats() because that would break
  critical "all workers report about the same stats" expectations of the
  special hack in `StoreInfoStats::operator +=()`.

To make both SNMP and cache manager use cases work, we removed the hack
from StoreInfoStats::operator +=() and hacked MemStore::getStats()
instead, making the first worker responsible for shared memory cache
stats reporting (unlike SMP rock diskers, there is no single kid process
dedicated to managing a shared memory cache). StoreInfoStats operator
now uses natural aggregation logic without hacks.

TODO: After these changes, StoreInfoStats::mem.shared becomes
essentially unused because it was only used to enable special
aggregation hack in StoreInfoStats that no longer exists. Remove?