[chord] merkle - terminate db iterator

[Benjie Chen]

so i reverted to a previous revision of database_get_keys, as josh and
james both suggested, where we terminate the db iteration after the range.
provided db3 does the right now and return keys in order, this fix does
the job and produces the following performance:

trial 0  - 7.8
trial 1  - 8.0
trial 2  - 8.2
trial 3  - 8.5
trial 4  - 8.1  
trial 5  - 8.1
trial 6  - 7.8
trial 7  - 7.5
trial 8  - 8.2

so this looks good, and we don't have to keep an array of keys.
i'm happy with this. thanks for helping me get this resolved.

benjie

-- 
benjie chen
benjie at lcs.mit.edu