genomics: use larger messages in sequence stores

When an actor sends a message to another actor, there are basically
3 types of routes (this is hidden inside Thorium):

1) destination actor is on the same worker;
2) destination actor is on the same node, but on a difference worker);
3) destination actor is on another node.

biosal_sequence_store actors, biosal_assembly_arc_kernel actors, and
biosal_arc_classifier actors are distributed such that there is a 1 of each
on each worker (1:1:1). Right now, sending to an actor on the same worker
is as slow as sending a message to an actor on the same node, but on a different
worker.

Increasing the buffer size of messages sent by biosal_sequence_store actors
basically lowers the number of messages required to fill the bins in
the corresponding biosal_assembly_arc_classifier actor.

This changeset increases the buffer size from 256 KiB to 2 MiB (8X larger).

The tests performed on the IBM Blue Gene/Q also points out that these
2 tickets are important:

use the multiplexer only on specific messages
https://github.com/GeneAssembly/biosal/issues/749

avoid going through node when destination actor is on the same worker
https://github.com/GeneAssembly/biosal/issues/748

In particular, messages with 0 bytes should not have to wait for the multiplexer
timeout since they are typically replies to some random requests.

[boisvert@cetuslac1 biosal-tests]$ grep TIMER soil-747-1024x16-9.output
TIMER [Load input / Count input data] 44.024097 seconds
TIMER [Load input / Distribute input data] 1 minutes, 52.367249 seconds
TIMER [Load input] 2 minutes, 36.391342 seconds
TIMER [Build assembly graph / Distribute vertices] 11 minutes, 4.401550 seconds
TIMER [Build assembly graph / Distribute arcs] 24 minutes, 17.566284 seconds
TIMER [Build assembly graph] 35 minutes, 21.967773 seconds

Link: https://github.com/GeneAssembly/biosal/issues/747
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>