Summary:
List of changes:

1) Fix the snprintf() usage in cases where wrong variable was used to determine the output buffer size.

2) Remove unnecessary checks before calling delete operator.

3) Increase code correctness by using size_t type when getting vector's size.

4) Unify the coding style by removing namespace::std usage at the top of the file to confirm to the majority usage.

5) Fix various lint errors pointed out by 'arc lint'.

Test Plan:
Code review and build:

git diff
make clean
make -j 32 commit-prereq
arc lint

Reviewers: kradhakrishnan, sdong, rven, anthony, yhchiang, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D51849