Changes to support RockDB examples and RocksDB on YCSB

[OmSaran]

Changes to support RocksDB

This includes the following changes

1. Honour O_CLOEXEC in open call
2. FCNTL honour O_CLOEXEC
3. Redirect FREAD_UNLOCKED to FREAD
4. Handle 'pread' calls in SplitFS
5. Intercept fallocate
6. Intercept sync_file_range
7. Add unit tests for the new calls implemented
8. Add implementation.md to document the new calls.
9. Added workaround for a corner case issue in relink system call where the file size is bigger than expected after relink.
10. Add RocksDB source files.
11. Add YCSB scripts to run it on RocksDB
12. Update README.md and experiments.md with corresponding changes.