Make pool size parameters configurable

[fquinner]

Including number of nodes to allocate and the size of each node.

[fquinner]

commit e3c6343
Author: Frank Quinn fquinn.ni@gmail.com
Date: Thu Jun 2 21:41:48 2016 +0100

Added new configuration parameters for message pool and node sizes

I provided sample usage in the configuration:

mama.zmq.transport.sub.msg_pool_size=1024
mama.zmq.transport.sub.msg_node_size=4096

Pool size is in number of nodes and node size is in bytes. A larger
initial pool will mean fewer allocations if the subscriber is likely
to be placed under pressure.