WL#1798: Make minimum keyblock settable as variable

Currently minimum key block size (1K) is hardcoded while it should be settable
as server startup variable.

The task WL#602 is extending this task by allowing value setting on Index and
Table level.

Setting minumum key block size is very useful in 2 reasons

1) To make LOAD INDEX work which currently does not work if indexes have different 
   block size (workaround)
2) To have IO in OS_PAGE size blocks which are usually 4K. 

According to the Igor this task should be rather easy to implement.