fix a few compile warnings

1, const qualifiers on return types make no sense and will trigger a compile warning: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]

2, class HistogramImpl has virtual functions and thus should have a virtual destructor

3, with some toolchain, the macro __STDC_FORMAT_MACROS is predefined and thus should be checked before define

Change-Id: I69747a03bfae88671bfbb2637c80d17600159c99
Signed-off-by: liuhuahang <liuhuahang@zerus.co>
Merge pull request #269 from huahang/patch-2

fix a few compile warnings
Relax backupable rate limiting test
Fix travis builds

Summary:
Lots of travis builds are failing because on EnvPosixTest.RandomAccessUniqueID: https://travis-ci.org/facebook/rocksdb/builds/34400833

This is the result of their environment and not because of RocksDB's bug.

Also note that RocksDB works correctly even though UniqueID feature is not present in the system (as it's the case with os x)

Test Plan:
OPT=-DTRAVIS make env_test && ./env_test
Observed that offending tests are not being run

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22803
created a new ReadOptions parameter 'iterate_upper_bound'