MINOR: debug: make ha_stuck_warning() print the whole message at once

MINOR: debug: make ha_stuck_warning() print the whole message at once

It has been noticed quite a few times during troubleshooting and even
testing that warnings can happen in avalanches from multiple threads
at the same time, and that their reporting it interleaved bacause the
output is produced in small chunks. Originally, this code inspired by
the panic code aimed at making sure to log whatever could be emitted
in case it would crash later. But this approach was wrong since writes
are atomic, and performing 5 writes in sequence in each dumping thread
also means that the outputs can be mixed up at 5 different locations
between multiple threads. The output of warnings is never very long,
and the stack-based buffer is 4kB so let's just concatenate everything
in the buffer and emit it at once using a single write(). Now there's
no longer this confusion on the output.