Fixes #1689

Needed for #1689

In context of #1691

* don't recycle input buffer if this leads to growth of the output
buffer

Correct misprint in tarantool.rb

Relates to https://github.com/tarantool/tarantool/issues/1497#issuecomment-220993800

Speed up acceptor with enabled io_collect_interval.

 tarantool-lts-client (1.5) and tarantool (1.6)
 have file /usr/bin/tarantool

When selecting the next buffer to flush, take into account not
just its size, but how much of it has already been flushed.
This saves a couple syscalls per connection per event loop.

Replace socket('AF_UNIX', 'SOCK_STREAM', 'ip') with
        socket('AF_UNIX', 'SOCK_STREAM', 0).

Fixes distributions with missing "ip 0 IP" line in /etc/protocols
(e.g. Arch Linux).

 on memory fail there could be port->reply.found > 0 and uninitialized port->svp,
 that could lead to random error after svp restoration.
*) possibly fixed OPENTAR-97

closes gh-856

Rename global `fiber` into `fiber_ptr`

Rename global `fiber` into `fiber_ptr` (for use with gdb and avoidance of ambiguity with `struct fiber`)

Backport fix from tarantool/mysql by Anton Reznikov

Also fix :send() and :write()