Limit maximum number to loop to 100000

if we need that many iterations, most probably run time is already
bound by the loop itself
ENH: by default no repeat or number specified -- deduce both targetting .5sec target timing

having only 3 repeats by default to take min of adds to variance.
Deduce both repeat and number (if none specified -- sqrt of total number of runs needed
to meet target timing).  It seems also to allow reduce 'looping' overhead thus
getting lower timing estimates for relatively fast statements
TST: for preceeding changes to magic_timeit
ENH: db -- get/delete benchmarks results for a revision
ENH: allow to rerun all the benchmarks while taking "min" with previous results

This, after few runs, should greatly reduce variability across the results
thus providing us more stable timelines of performance
ENH: reduce target_timing x5 -- to 100ms