iproto: change IPROTO_NOP code from 11 to 12

11 was initially used for SQL EXECUTE in 1.8, but 1.7 commit
b73030f2 ("iproto: add IPROTO_NOP request type") reassigned
it to NOP so after the merge SQL EXECUTE landed at 12, which
broke connectors. Let's shift NOP to 12 and move EXECUTE back
to 11. This is OK as 1.7.7 which introduced the new iproto type
hasn't been officially released yet.