High throughput I/O (Multiple Ethernet Connections) - Rebase

[mundya]

A rebase of @mossblaser's #177

When finished, this PR fixes #142. This is a branch off the fault-tollerant-rig-ps branch and is intended for merging after #176.

• Facilities will be added to discover (and use) multiple Ethernet connections
• A set of read_into() methods will need to be added to allow creation of the
  read buffer in advance.
• The begin_burst function will simply set a flag.
• For each connection there will be a queue of read/write requests.
• All read/write requests should be wrapped in a lambda(?) and placed in the
  buffer. For conventional reads, a buffer should be created and then
  read_into called with that buffer.
• When not in a burst, the buffer will be emptied immediately (or a fast path
  used...).
• The end_burst function will spawn some threads and in each thread work on
  processing the queues.