gossip_cyclon: remove superfluous is_state type_checker

Fixes failing typecheck tests
memtest_SUITE: fix printing of error messages
gossip: fix bug when returing 'retry' select_data()

When returning retry on a select_data() call the trigger message was stored in
the message queue. When contiously returing 'retry' (e.g. in a one node setup with
gossip_cyclon), this led to a self-accelarting endless recursion of resending
and storing the trigger message.

This also fixes the failures in memtest_SUITE.