FIX: Make HeartBtInt configurable

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
FIX: Update timestamps of the last transmitted and received messages

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
FIX: Administrative level messages treatment

The patch consists of two parts:

* fix_session_admin() function is introduced. It both handles a subset
  of the administrative messages and keeps track of the incoming message
  sequence numbers. The function returns 0 if it was able to process a
  message, a user should not process the message further, 1 - if the
  function was not able to handle a message, a user should decide on
  what to do further

* fix_session_keepalive() function s introduced. Its purpose is to
  maintain connection status by generating activity if nothing was
  received/transmitted in a specified time. Further details can be read
  in FIX specification, namely: conditions on which HeartBeats and
  TestRequests are issued

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
[ penberg@kernel.org: Use LDFLAGS for -lrt to fix 64-bit Fedora issue. ]
Signed-off-by: Pekka Enberg <penberg@kernel.org>