Update to leveldb 1.6

Highlights
----------
Mmap at most 1000 files on Posix to improve performance for large databases.
Support for more architectures (thanks to Alexander K.)

Building and porting
--------------------
HP/UX support (issue 126)
AtomicPointer for ia64 (issue 123)
Sparc v9 support (issue 124)
Atomic ops for powerpc
Use -fno-builtin-memcmp only when using g++
Simplify IOS build rules (issue 114)
Use CXXFLAGS instead of CFLAGS when invoking C++ compiler (issue 118)
Fix snappy shared library problem (issue 94)
Fix shared library installation path regression
Endian-ness detection tweak for FreeBSD

Bug fixes
---------
Stop ignoring FLAGS_open_files in db_bench
Make bloom test behavior agnostic to endian-ness

Performance
-----------
Limit number of mmapped files to 1000 to improve perf for large dbs
Do not delay for 1 second on shutdown path (issue 125)

Misc
----
Make InMemoryEnv return a no-op logger
C binding now has a wrapper for free (issue 117)
Add thread-safety annotations
Added an in-process lock table (issue 120)
Make RandomAccessFile and SequentialFile non-copyable
Small fixes.

Details:
* Fix shared library building.
* Reorganize linking commands so flags like --as-needed can be passed.
* C binding exports version numbers.
* Fix small typos in documention.
added utility to dump leveldb files
Fix corruption bug found and analyzed by dhruba@gmail.com

https://groups.google.com/d/msg/leveldb/Kc9JxuIUu5A/9P0N9RL4ar8J
Merge remote-tracking branch 'google/master' into berge

* google/master:
  Fix corruption bug found and analyzed by dhruba@gmail.com
  added utility to dump leveldb files
  Small fixes.
  Update to leveldb 1.6