sql: remove sync from sql_request/response

sql_request is now decoded entirely in iproto thread
in iproto_msg_decode unlike other similar requests
like CALL or UPDATE which could be decoded in iproto
too.

But iproto thread pays for this optimization with a
leak - sql_request.bind array is allocated on iproto
thread region and never freed.

A fix is to decode, apply and free bind array in tx
thread in one place: tx_process_sql. For this
sql_request should look like other requests: do not
decode arrays, do not store sync, store fields as
raw MsgPack.

First step - remove sync.

Part of #3828