remove tailing_iter.h/cc

Summary: as title

Test Plan:
make all check
ran db_bench and saw seek stats at the end

Reviewers: yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21651
Fixed a signed-unsigned comparison error in db_test

Summary:
Fixed a signed-unsigned comparison error in db_test

Test Plan:
make db_test
Merge branch 'master' of github.com:facebook/rocksdb into push-compaction
Fixed compile errors (signed / unsigned comparison) in cuckoo_table_db_test on Mac

Summary:
Fixed compile errors (signed / unsigned comparison) in cuckoo_table_db_test on Mac

Test Plan:
make cuckoo_table_db_test
Merge branch 'master' of github.com:facebook/rocksdb into push-compaction