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r746: tuned heuristic for GRCh38

Reduced -c to 500 by default. As a compensation, we choose up to 1000 positions
if a seed has 500 or more occurrences. In addition, a read with big portion
from such seeds will have lower mapping quality.
r747: fixed a minor issue in the last (mis)commit
r748: reduced the default -m to 50
r749: max_hits tunable on CMD; default to 5
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