Add LockHeader()/StopConcurrentMutations() functions to reduce repeated code/comments in TcmallocSlab.
Add LockHeader()/StopConcurrentMutations() functions to reduce repeated code/comments in TcmallocSlab.

Also:
- Simplify the Locking code in ShrinkOtherCache().
- Move the order of declarations in TcmallocSlab's private section to match the style guide: types, static constants, functions, data members.
PiperOrigin-RevId: 418682433
Change-Id: I27cdb88eeb21fc686f5ec177e3cea8aaa85bf31e

google/tcmalloc,google/tcmalloc,google/tcmalloc