Eliminate stdout message when launching a posix thread.

This seems out of place as it's the only time RocksDB prints to stdout in the
normal course of operations. Thread IDs can still be retrieved from the LOG
file: cut -d ' ' -f2 LOG | sort | uniq | egrep -x '[0-9a-f]+'
Merge pull request #58 from mlin/no-stdout

Eliminate stdout message when launching a posix thread.
Fix printf format
use super_version in NewIterator() and MultiGet() function

Summary:
Use super_version insider NewIterator to avoid Ref() each component
separately under mutex
The new added bench shows NewIterator QPS increases from 515K to 719K
No meaningful improvement for multiget I guess due to its relatively small
cost comparing to 90 keys fetch in the test.

Test Plan: unit test and db_bench

Reviewers: igor, sdong

Reviewed By: igor

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D15609
Merge branch 'master' into columnfamilies

Conflicts:
	db/db_impl.cc
	db/db_impl.h
	db/db_impl_readonly.cc