Allow class Compaction to handle input files from multiple levels.

Summary:
Allow class Compaction to handle input files from multiple levels.
This diff is a subset of https://reviews.facebook.net/D19263 where
only db/compaction.cc and db/compaction.h are changed.

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19923
Guarding files_ attribute with #ifndef NDEBUG guard in FilePicker class.

Summary: Adding guards to files_ attribute of FilePicker class. This attribute is used only in DEBUG mode. This fixes build of static_lib in mac.

Test Plan:
make static_lib in mac
make check all in devserver

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D20163
Fixed a signed and unsigned comparison in Compaction

Summary:
Fixed a signed and unsigned comparison in Compaction

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test
Add -Wsign-compare to WARNING_FLAGS in Makefile

Summary:
Add -Wsign-compare to WARNING_FLAGS in Makefile as not all g++ compiler
include -Wsign-compare in -Wall when compiling '.h' file.

Test Plan: make -j32

Reviewers: ljin, igor, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D20169
Add MaxInputLevel() to CompactionPicker

Summary:
Having if-then branch for different compaction strategies is considered
hacky and make CompactionPicker less pluggable.  This diff removes two
of such if-then branches in version_set.cc by adding MaxInputLevel() to
CompactionPicker.

    // Given the current number of levels, returns the lowest allowed level
    // for compaction input.
    virtual int MaxInputLevel(int current_num_levels) const;

Test Plan:
make db_test
export ROCKSDB_TESTS=Compaction
./db_test

Reviewers: igor, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19971