#1097 reduce memory overhead of the group by data structures

avoid synthetic access methods

since it seems to be more efficient than repeated lookups

was deceived by the variability of the test

pretty close to the hsqldb performance, in fact, 20 seconds for H2 vs 17 seconds for HSQLDB

and add remove() method for Java1.7 compat

speeds up this query by about 30%

now the test is both faster and uses less memory than originally

Add some protection to ValueHashMap against hashes with the same less significant bits

originally from Evgenij Ryazanov

Add external storage of temporary results for MVStore mode