[RFC] SMPP enquire link

Hi All

We are experiencing the following problem with the SMPP module.

There are 2 instances of io_thread
sender: - calls send_enquire_link and send_messages
receiver: - calls send_enquire_link, after each PDU it reads and handles.

What happens when there are a largish number of messages in the 
msgs_to_send list,
1. the send_messages loop runs and we end up with a situation where one 
needs to throttle the throughput to the SMPP server
2. the send_message loop now sleeps
3. the io_thread doing send_messages now cannot call send_enquire_link 
regularly enough to check whether it is time to send an enquire link to the 
SMPP server.
4. and sometimes we havent yet started to receive submit_sm's back from the 
smpp server - so send_enquire_link is not getting called by the io_threads..


I propose that we make an extra thread for the enquire_link sender. A very 
simple thread that will  simply send and enquire_link and then sleep. There 
will be 2 instances of this thread, one for each of the instances of 
io_thread. io_thread will be responsible for starting and stopping this thread.

Nisan