crimson/osd: tune the default for crimson_alien_op_num_threads.

crimson/osd: tune the default for crimson_alien_op_num_threads.

This commit tries to optimize the default size of AlienStore's
thread pool. Recent testing has shown the intial value, which
is `16`, is far from being optimal while the optimum is close
to `4`:

  https://gist.github.com/rzarzynski/c851212f4b0baef4097a6087533ba17b

Further testing by Mark Nelson suggests that the sweet spot is
closer to `8` taking into account 4 KB writes with lower queue
depths:

  https://docs.google.com/spreadsheets/d/1IR9ysWRkaGdX5e9w_YV8kfeeglR2pEY3V9hovY0ECV8/edit#gid=1884533182

As a kind of compromise `6` is proposed in this commit.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>