Phase 1 of an iterator stress test
Phase 1 of an iterator stress test

Summary:
Added MultiIterate() which does a seek and some Next/Prev
calls.  Iterator status is checked only, no data integrity check

Test Plan:
make db_stress
./db_stress --iterpercent=<nonzero value> --readpercent=, etc.

Reviewers: emayanke, dhruba, xjin

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12915

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