MINOR: server: yield during "purge all servers"

MINOR: server: yield during "purge all servers"

Extend "purge all servers" handler to yield after 50 iterations. The
objective is to avoid to consume too much time under thread isolation.

When yielding must be performed, applet_have_more_data() is manually
called. This is necessary as the applet is not blocked on buffer output.
Without it, CLI handler won't be called again.