rewriting [box-protocol.rst]

Add diagrams/rewrite spec. Add snaps/xlog and replication commands overview.
Refactor access check headers.

Make sure the tests pass under low memory and no memory overcommit.
Fix a broken travis build (missing pthread library when linking a test).
Rename user_cache_find to user_by_id().

Rename user_cache_find to user_by_id() to be consistent
with func_by_id(), space_by_id() - API variant which
does not throw exceptions if an object is not found.
Minor code refactoring.

Encapsulate error handling in user cache search functions.
New epoch of parallel testing. Brand new.with blackjack and h*****s. In fact, forget the testing\!
Fixes, Refactoring
Embedding testing system into the core of test-run.py