Fix db_write_test clang/windows build failure

Summary:
Fix db_write_test clang/windows build failure. Explicitly cast size_t (unsigned long) to uint32_t (unsigned int).
Closes https://github.com/facebook/rocksdb/pull/2407

Differential Revision: D5182995

Pulled By: yiwu-arbug

fbshipit-source-id: aba225a9fccb12d5bfbdc2cd6efc11040706a9d2

fixed typo

Summary:
fixed typo
Closes https://github.com/facebook/rocksdb/pull/2376

Differential Revision: D5183630

Pulled By: ajkr

fbshipit-source-id: 133cfd0445959e70aa2cd1a12151bf3c0c5c3ac5

Fix the Java build which was broken by a4d9c02

Summary: Closes https://github.com/facebook/rocksdb/pull/2406

Differential Revision: D5181091

Pulled By: ajkr

fbshipit-source-id: fd72525da4fb1d50143080a210f8d824cbb968d6

remove test dir before exit when current regression is running

Summary:
clean up the current test dir if the last regression test is still running.
Closes https://github.com/facebook/rocksdb/pull/2401

Differential Revision: D5177882

Pulled By: lightmark

fbshipit-source-id: 91d899fcc2bde841948eae71af8584d4bdb35468