Make the THD class have pointers to the Protocol_text and
Protocol_binary instances, so that sql_class.h doesn't have to include
protocol.h and protocol_classic.h.

Move the Query_fetch_protocol_binary class definition from
sql_prepare.h to sql_prepare.cc, since it is only used in that file.

Move PTI_simple_ident_q_3d::itemize() from parse_tree_items.h to
parse_tree_items.h, so that the header doesn't need to include
protocol.h.

Include plugin.h instead of protocol.h from thd_var_access.h, as it is
only used to get the definition of MYSQL_THD.

Change-Id: Iecc1b4986b45533d9b2187e7a25d6c4dd932f6f0