Summary: Pull Mark's patch and slightly revise it. I revised another place in db_impl.cc with similar new formula.

Test Plan:
make all check. Also run "time ./db_bench --num=2500000000 --numdistinct=2200000000". It has run for 20+ hours and hasn't finished. Looks good so far:

Installed stack trace handler for SIGILL SIGSEGV SIGBUS SIGABRT
LevelDB:    version 2.0
Date:       Tue Aug 20 23:11:55 2013
CPU:        32 * Intel(R) Xeon(R) CPU E5-2660 0 @ 2.20GHz
CPUCache:   20480 KB
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    2500000000
RawSize:    276565.6 MB (estimated)
FileSize:   157356.3 MB (estimated)
Write rate limit: 0
Compression: snappy
WARNING: Assertions are enabled; benchmarks unnecessarily slow
------------------------------------------------
DB path: [/tmp/leveldbtest-3088/dbbench]
fillseq      :    7202.000 micros/op 138 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
fillsync     :    7148.000 micros/op 139 ops/sec; (2500000 ops)
DB path: [/tmp/leveldbtest-3088/dbbench]
fillrandom   :    7105.000 micros/op 140 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
overwrite    :    6930.000 micros/op 144 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
readrandom   :       1.020 micros/op 980507 ops/sec; (0 of 2500000000 found)
DB path: [/tmp/leveldbtest-3088/dbbench]
readrandom   :       1.021 micros/op 979620 ops/sec; (0 of 2500000000 found)
DB path: [/tmp/leveldbtest-3088/dbbench]
readseq      :     113.000 micros/op 8849 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
readreverse  :     102.000 micros/op 9803 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
Created bg thread 0x7f0ac17f7700
compact      :  111701.000 micros/op 8 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
readrandom   :       1.020 micros/op 980376 ops/sec; (0 of 2500000000 found)
DB path: [/tmp/leveldbtest-3088/dbbench]
readseq      :     120.000 micros/op 8333 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
readreverse  :      29.000 micros/op 34482 ops/sec;
DB path: [/tmp/leveldbtest-3088/dbbench]
... finished 618100000 ops

Reviewers: MarkCallaghan, haobo, dhruba, chip

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D12441