sql: move sql_request and xrow_decode_sql to xrow lib

All binary struct *_request are stored in xrow.h/.c
together with their decoders. The only reason why
xrow_decode_sql was implemented in execute.c was a
dependency on struct sql_bind. Now xrow_decode_sql
and struct sql_request operate only by MessagePack and
some iproto constants and are moved to their true
home.

Follow up #3828