iproto: clear request::header for client requests

To apply a client request, we only need to know its type and body. All
the meta information, such as LSN, TSN, or replica id, must be set by
WAL. Currently, however, it isn't necessarily true: iproto leaves a
request header received over iproto as is, and tx will reuse the header
instead of allocating a new one in this case, which is needed to process
replication requests, see txn_add_redo().

Unless a client actually sets one of those meta fields, this causes no
problems. However, if we added transaction support to the replication
protocol, reusing the header would result in broken xlog, because
currently, all requests received over iproto have the is_commit field
set in xrow_header for the lack of TSN, while is_commit must only be set
for the final statement in a transaction. One way to fix it would be
clearing is_commit explicitly in iproto, but ignoring the whole header
received over iproto looks more logical and error-proof.

Needed for #5860