Add test for one command in a transaction
Add static helper for setting up a transaction in the buffer
Fixup for deserializer_state(_new())
Fixup deserialize_state_new()
Fixup deserializer_state struct
Fixup for ws_transaction_push_statement()
Fix ugly and buggy code for argument appending

This code was buggy, also after the fixing merge. Sometimes it is better
to write more code to be clear what happens when.