Use IterKey instead of string in Block::Iter to reduce malloc

Summary:
  Modify a functioin TrimAppend in dbformat.h: IterKey. Write a test for it in dbformat_test
  Use IterKey in block::Iter to replace std::string to reduce malloc.

  Evaluate it using perf record.
  malloc: 4.26% -> 2.91%
  free: 3.61% -> 3.08%

Test Plan:
  make all check
  ./valgrind db_test dbformat_test

Reviewers: ljin, haobo, yhchiang, dhruba, igor, sdong

Reviewed By: sdong

Differential Revision: https://reviews.facebook.net/D20433