Don't use palloc()/ptruncate() in box_lua.cc.
This is a pre-requisite patch for tuple-format.
tuple:transform() method remains to be fixed in a separate patch.

- make the test suite actually paste
- remove the ugly copy-paste

Use an existing method to pack a key into a tbuf.
@todo: switch to using a list of keys instead of a buffer
for expire.

- introduce primary_key_validate
- make asserts follow the coding style
- the original patch introduced a regression
in the error message when using a multi-part
key in update/delete: since, in the original patch,
key_validate was invoked before primary-key validation,
the error message changed to a less suitable one. Remove
this.
- an assert that something is not NULL is useless noise,
since a few lines down there's definitely going to be
NULL pointer dereference.

Rename tarantool_pthread.h to tt_pthread.h

This is a pre-requisite for addition of Perl stored procedures.

Minor style edits.

@todo: write the reason why we skip a test when we skip it.