Add an uint64 test flag `min-blob-size` for Titan in titandb_bench and titandb_stress tests.

Adding scripts/format-diff.sh to use clang-format to format code changes since diverge from master. Also adding travis CI job to check if the code needs to format. Also format all existing code.
Signed-off-by: Yi Wu <yiwu@pingcap.com>

As Rocksdb described, `DropColumnFamilies()` only records the drop of the column family specified by ColumnFamilyHandle. The actual data is not deleted until the client calls `delete column_family`, namely `DestroyColumnFamilyHandle()`. We can still continue using the column family if we have outstanding ColumnFamilyHandle pointer.	

now there are some problems:

- after dropping column family, the blob files of it may be deleted before column family handle is destroyed.
- When opening DB without some existing column families(not supported yet), the blob file of these will be regarded as obsolete files and deleted. 
- After destroyed column handle(not drop the column, just not need to read/write it anymore), we need to remove inner blob storage and evict related cache.

use `lcov` to generate code coverage report together with GCC build, and send result to codecov. Also some minor refactor to travis config.
Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Connor1996 <zbk602423539@gmail.com>

Update cmake script to add jemalloc and compression libraries' include directories. This is to fix missing headers in build, e.g: https://travis-ci.org/pingcap/rust-rocksdb/jobs/529604990

After the fix rust-rocksdb build will pass: https://travis-ci.org/yiwu-arbug/rust-rocksdb/jobs/529906506

Adding travis config to trigger CI build. Adjust cmake script accordingly.

Signed-off-by: Yi Wu yiwu@pingcap.com

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Summary:
Adding a cmake script so we can build static lib for Titan:
* build static lib libtitan.a, titandb_bench, titandb_stress, and unit tests.
* the script should generate the same compile and link flags (by copying part of rocksdb/CMakeLists.txt into cmake/rocksdb_flags.cmake)
* See the new README.md file for build script usage

Other changes:
* updated README.md
* updated header paths
* remove titan_c.h and titan_c.cc, which is not being used anywhere
* add a .gitignore

Test Plan:
```
mkdir -p build; 
cd build; 
cmake .. -DROCKSDB_DIR=../rocksdb;
make -j
ctest -R titan
```

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Signed-off-by: Yi Wu <yiwu@pingcap.com>