TRAFODION-1789 & TRAFODION-1812TRAFODION-1789: Create the histogram metadata tables at schema creation timeAdded a list of MDTableInfo structs called allMDHistInfo that describeshistogram tables similar to other metadata tables in CmpSeabaseDDLmd.h.During schema create, this list is traversed and each histogram table created.This is performed by calling the static function - createHistogramTables.Similarily, this list is traversed at drop schema time to drop existinghistogram tables. With this change, you can drop a schema without thecascade option when only histogram tables remain.In histogram code (hs_cli.cpp), removed hard coded create statements andchanged code to call the createHistogramTables static function. With thischange, there is only one place where Histogram tables are defined. Today,histogram tables are defined in ../sqlcomp/CmpSeabaseDDLmd.h and../ustat/hs_cli.cppTRAFODION-1812: cleanup schema causes transaction to hang in aborting stateWhile testing the fix for TRAFODION-1789, some of the regression tests failed.There is an issue when trying to cleanup a schema that contains histogramtables. Changes were made to cleanup histogram tables first and to avoiddeleting histogram entries for the histogram table itself.Added IF EXISTS syntax to routine drops. Also added IF NOT EXISTS syntax toroutine creates. The DROP IF EXISTS was needed to fix an issue when droppingroutines as part of drop schema code.