BUG/MEDIUM: stream: use non-blocking freq_ctr calls from the stream dumper

BUG/MEDIUM: stream: use non-blocking freq_ctr calls from the stream dumper

The stream dump function is called from signal handlers (warning, show
threads, panic). It makes use of read_freq_ctr() which might possibly
block if it tries to access a locked freq_ctr in the process of being
updated, e.g. by the current thread.

Here we're relying on the non-blocking API instead. It may return incorrect
values (typically smaller ones after resetting the curr counter) but at
least it will not block.

This needs to be backported to stable versions along with the previous
commit below:

   MINOR: freq_ctr: provide non-blocking read functions

At least 3.1 is concerned as the warnings tend to increase the risk of
this situation appearing.