more efficient smaller files

- smallfiles start with 1mb instead of 16mb files
- bugfix: space at end of files was never used, so sometimes big gap of
wasted space (in case of big extents)! Fixed: each file is fully used
(free space bigger than needed space but lower than next calculated
extent size)
- using smaller extents, otherwise very big extents were allocated for
small needed allocs -> unnecessary big files
renamed project "mongod" to "minimongod"

different executable name to distinguish between "normal" mongo