optimizing index sampling for better memory usage

[Radim Kolar <hs...@sendmail.cz>]

currently j.o.a.c.io.sstable.indexsummary is implemented as ArrayList of 
KeyPosition (RowPosition key, long offset)
i propose to change it to

RowPosition keys[]
long offsets[]

this will lower number of java objects used per entry from 2 
(KeyPosition + RowPosition) to 1.

For building these arrays convenient ArrayList class can be used and 
then call to .toArray() on it.