when actually preforming a multi-statement select with any non-trivial
components, the `next_result` call will block the ruby interpreter.
We fix this in two ways:

First, we wrap up the actual mysql_next_result call in a "no-gvl"
call so that the interpreter can do other work while next_result is
happening.  Second, we borrow a method from do_query and spin in a
select() loop on the mysql socket until the query is ready.  A quick
source dive into the underlying mysql source shows that next_result()
falls into the same path as mysql_real_query (cli_read_query_result), so
this should be a safe method.

Rather than including violite.h to be able to look into the net.vio structure
to call its has_data function pointer, we borrow the definitions of just two
functions that understand this structure.

The underlying client library may have pre-read the results from the next
query, so wait_for_fd would not be triggered. Instead we will ask whether
the net.vio structure has additional data that hasn't been parsed.

Hack: use knowledge of whether the connecti is SSL or not to call the
appropriate has_data function and pass net.vio as an opaque structure.

Definitions are from vio_priv.h