When I exit SSH session bitcoind goes down
You could use screen or a similar tool like tmux or byoblu. 
Otherwise use the NOHUP command. 

nohup is a POSIX command to ignore the HUP (hangup) signal. The HUP signal is, by convention, the way a terminal warns dependent processes of logout.
Output that would normally go to the terminal goes to a file called nohup.out if it has not already been redirected.
nohup is a low-level utility simply configuring a command to ignore a signal. As seen below, nohup is very far from being a full-featured batch system solving all the problems of running programs asynchronously.


  $ nohup abcd &amp;
  
  $ exit


Alternatives:
The screen or tmux command can also detach a process from the current shell. It allows to reattach to the process later on.
e.g. the following call will run somescript.sh detached from SIGHUP and in the background:


  $ screen -A -m -d -S somename ./somescript.sh &amp;


https://en.wikipedia.org/wiki/Nohup

My advice is to learn to use screen (or tmux) so you can later re-attach the process to the main shell and interact with it.