Fix some bugs in the function (now it uses libc's (3) dirname ffi.call).

A dropped user structure could still be accessed from sessions
which were authenticated with this user.

Check that a user is dropped when accessing it from the authenticated
session.

 + Add tarantoolctl enter instance_name command

This patch also changes box.info.server output for this case.

Avoid uncontrolled growth of region memory.

Stop producing "excessively sparse array"

Other changes:

 - "" or NULL=ffi.cast('void *', 0) now resets option to default value
 - app/cfg.test.lua converted to TAP format

Tarantool started to use getaddrinfo() to find suitable address for
binding both for IPROTO and CONSOLE ports. A hostname can be resolved to
various addresses families, depending on user settings. For example, all
moderns systems resolve 'localhost' to '::1' by default. This creates a
problem if one subsystem binds to '127.0.0.1' and some other tries to
connect '::1'. This patch unify hardcoded names and addresses in entire test
suite to use the same value. This is pre-requisite for #447, #368 and other
URI-related patches.

Other changes:

  * console.listen() now properly handles address arguments
  * console.listen() re-based on socket.tcp_server() code

socket.tcp_server() changes:

 * Set nice fibers names for listener and clients
 * Use only one address for bind
 * Return single socket instead of wrapper
 * Add opts = {} to arguments
 * Automatically remove UNIX domain socket on server:close()

Other changes:

 * Fix buggy port number parsng for ipv6
 * Add peer address to accept() results like recvfrom()

memtx_arena is shared memory, so TreeIndex can't use it as is
(breaks snapshotting).

The wal writer now allocates memory to encode a binary log row,
but never used to free it. Add fiber_gc().

Restore fiber_gc() in recovery, and also move index allocation
from runtime to memtx arena.

Minor fixes & cleanups.

 * Split parser and resolver in src/uri.rl
 * Extend URI parser grammar to fully support RFC3986
 * Add Lua bindings to URI parser
 * Implement proper DNS logic in coio_connect() /
   evio_service_bind()
 * Replace uri_to_string() with sio_strfaddr()

Prerequisite for #447

Drop redundant enums for IOVMAX.
Better names for message encode/decode functions.

xrow.h - a universal representation of Tarantool binary request/response,
either in iproto protocol (binary client/server protocol),
snapshot or binary log (xlog).

Add a throwaway spot memory pool to recovery.