crimson: make the number of alien threads configurable.

crimson: make the number of alien threads configurable.

It's particularly important for reads to have large-enough
thread pool in `AlienStore` as they are synchronous; that
is, e.g. `BlueStore` blocks on IO when handling reads.

This commit introduces a configurable allowing operators
to increase the number from `1` which we were limited to
before the change.

Naming similarity with `osd_op_num_threads_per_shard` is
intentional.

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