txn_rollback started to yield recently. Since it was invoked via a ffi
call, which must never yield, all sorts of bad things started to happen.

* the bug with iobuf recycling has always been in 1.6,
  and is fixed in 1.7, but got more exposure with the patch
  for gh-846. "Idle" state of iobuf was calculated incorrectly.

Fixes #810

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

tx_process_join_subscribe() attempted to write response to closed socket.
This patch also fixes diag_raise() with SocketError.

Fixes gh-1624.

Fixes debian/#831361

Free the vclock_to_string() buffer in all cases.

Review fixes:
* revert changes in console.lua, they should be pushed down to
  net.box (it should accept empty passwords)

Previously the URI BNF didn't allow to specify empty
password. There should have been at least one symbol.

Now both user@host and user:@host forms are permitted.

In case of user@host, the password will parsed as nil.
In case of user:@host, the password will be parsed as
empty string.

Fixes #1512

Regression introduced by 9036eeeb

Fill IPROTO_KEY for DELETE and IPROTO_TUPLE for INSERT/REPLACE.

Delete the previous checkpoint in sophia only if we have a new one.

Actually fix the bug and add a test case.