BUGFIX invalid NULL check in hyperloglog_add_item_agg2
FIX 9.3 compatibility issue (no parameter names with CREATE AGGREGATE)
estimator enhancements - support for merging, operators, version bump

merging - all estimators except 'bitmap' support merging, i.e. computing
          estimators for parts of a set, and then combining them into
          a single one (for the set as a whole)

operators - added '#' (computing estimate) and '||' (merging estimators)