Fix bug https://bugs.launchpad.net/tarantool/+bug/990728
"box.space:truncate() is a CPU hog".

The old truncate algorithm would restart iteration
from the beginning to delete every tuple.
With HASH index it was especially slow, since
it would have to skip more and more empty slots
to find a non-empty slot for deletion.

Rewrite the truncate() to walk linearly
over all elements and delete them.

This works both with HASH and TREE indexes.

On the same token make truncate multipart-index
aware (was missed in multipart-pk patch).
Add a test for that.

To work with multipart keys efficiently,
implement tuple:slice(). A slice returns
a linear set of fields from a tuple.
Convenient to retreive a multipart key or
tuple tail. Document tuple:slice().

Make snap_io_rate_limit and io_collect_interval dynamic.
Add a test case.
Update the user guide.

Update the manual and the configuration file
to reflect the new WAL writer architecture.

https://bugs.launchpad.net/tarantool/+bug/837484

This reverts commit 8dc70aac.

and man file

- add content
- speed up rebuild by keeping static stuff in www-data
- change the layout and stylesheet

- removed SIGUSR2 forwarding
- removed logger and pid command line arguments
- moved man pages generation to doc/man/
- added logger_pid to show info output
- changed instance config preformating scheme