Make status() a method.
Update the docs.

Ensure a transaction is not left hanging around
when a session disconnects.

Add basic tests for transactions.

Conflicts:
	src/box/replica.cc
	src/box/txn.cc
	src/iproto_constants.cc
	src/iproto_constants.h
	test/box/bad_trigger.result
	test/box/call.result
	test/lib/tarantool-python

Implement transactions infrastructure for autocommit mode.
Prepare txn.cc for multi-statement transactions.

This patch changes iproto codes enumeration scheme:

* 0 - OK (indicates success result of DML or ADMIN commands)
* [    1, 128) - DML & ADMIN commands
* [32768, 65535) - errors (1<< 15 | ER_XXX from errcode.h)

Other ranges are reserved for future use.

Refactor statement transaction lifecycle. Avoid nested
statement transactions. Make sure there is no more than 1
statement transaction present in the system at the moment.

Resolve the bugs this revealed:
- in local hot standby we would parasite on the main or
script fiber session to process requests. When an assert
was added that there is no more than 1 transaction active
at a given moment in time, it turned out that transactions
running on behalf of local hot standby were running in
the scope of session of the run_script or sched fibers.

Create and use an own session there.

- refactor session on_connect/on_disconnect trigger
invocation: do not invoke these triggers for just any session,
only for real connects/disconnects (iproto, admin).

- split session creation/destruction and trigger invocation
Conclude that session storage implementation needs to be
done differently.

Every fiber which may work with box needs to have a session,
since it's stores user id and the current transaction.

Conflicts:
	src/lua/bsdsocket.lua
	test/box/bsdsocket.result
	test/box/bsdsocket.test

This patch adds a new implementation of UUID library.
New library generates RFC4122 compatible random-based (version 4)
UUIDs using data from /dev/(u)random. The uuidgen(2) syscall is also
supported on BSD system. `struct tt_uuid` redefined from RFC4122.
Complimentary functions rewrotten to remove dependecy on e2fs libuuid.
Lua bindings refactored to use new library and support all features.

Closes #317