Cover mistakes found while implementing
https://github.com/ziontab/lua-nginx-tarantool

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Misc cleanups & renames.

Fix a bug in sio_writev_all with possible
going beyond allocated memory.

Log file/line in system errors.

If replication handshake failed, panic, don't try to
reconnect and handshake again.

This will simplify client library construction cause it will allow to use
streaming MsgPack decoding.
If reply has variable part count, then request decoder which uses streaming
decoder, need to wait next part to analize: is it belongs to this reply
(body map) or start of next reply (ie length integer)? So that, if request
(ping) were single request, request decoder will stuck.

Unit test coio_stat.

Cleanup unit tests a bit: merge unit.h and test.h, which
would duplicate each other.

Fix a bug in trigger_atexit.test, which would not clean up after
itself.

Actually print an error message and class name in the log.

MP_INT is a format description, which allows value
in a negative range as well. In cases when we expect
non-negative values, it should be specified. Proclaiming
that MP_INT only allows unsigned integers is misleading.

Turn back on garbage collection during recovery from the binary log,
which was turned off when recovery was made part of the interactive mode.

Now there is no conflict between fiber gc pool usage in recovery
and in the interactive console, since the interactive console is pure
Lua.

Set xlog::vclock for write mode logs as well.
Move the check for matching signature into xlog_read_meta().
Convert xlog_open_stream() to use exceptions.