* Introduce multigeneration aggregation
* Do not save unused part of RGDatas to disk
* Add IO error explanation (strerror)
* Reduce memory usage while aggregating
* introduce in-memory generations to better memory utilization
* Try to limit the qty of buckets at a low limit
* Refactor disk aggregation a bit
* pass calculated hash into RowAggregation
* try to keep some RGData with free space in memory
* do not dump more than half of rowgroups to disk if generations are
allowed, instead start a new generation
* for each thread shift the first processed bucket at each iteration,
so the generations start more evenly
* Unify temp data location
* Explicitly create temp subdirectories
whether disk aggregation/join are enabled or not
MCS now chowns created directories hierarchy not only files and
immediate parent directories
Minor changes to cpimport's help printout
cpimport's -f option is now mandatory with mode 2
an owner for all data files created by cpimport
The patch consists of two parts: cpimport.bin changes, cpimport splitter
changes
cpimport.bin computes uid_t and gid_t early and propagates it down the stack
where MCS creates data files
