Removing StatisticsRegistry's static functions current() and registerStat().

Removing StatisticsRegistry's static functions current() and registerStat().
- The functionality the get the StatisticsRegistry attached to the SmtEngine was previously through StatisticsRegistry::current(). This is the dominant StatisticsRegistry in the code. (There is another StatisticsRegistry attached to the NodeManager.) Having this be a static function on StatisticsRegistry requires the use of an SmtEngine in the wrong compilation unit.
- Usages of StatisticsRegistry::current() that were visible in prop/{bvminisat,minisat} has been removed. A pointer to the relevant StatisticsRegistry should be passed instead into the constructor.
- The function StatisticsRegistry::current() has been replaced by SmtScope::currentStatisticsRegistry(). SmtScope is in the libcvc4 package, where SmtEngine is available in the compilation unit.
- The function smtStatisticsRegistry() is a synonym for SmtScope::currentStatisticsRegistry() in smt/smt_statistics_registry.h. This header has fewer include dependencies than the one for SmtScope.
- Correspondingly, the static functions StatisticsRegistry::{registerStat, unregisterStat} have been removed. One should instead use smtStatisticsRegistry()->{registerStat,unregisterStat} instead.
- The KEEP_STATISTIC macro has been moved into smt/smt_statistics_registry.h.
- Documents the reason StatisticsRegistry is CVC4_PUBLIC. This lets me remove the warning I added.
- Removing most operators for timespec from statistics_registry.h file. These a bit error prone in clang.
- Most of the really confusing ifdef's in util/statistics_registry.h are gone.