[LIVY-989][SERVER] Livy core support for interactive session idleTimeout

[askhatri]

What changes were proposed in this pull request?

Currently, a Livy interactive session has a field called ttl, which kills the session if it has been idle for a given amount of time. However, here is the expected behaviour:

ttl: kills the session if it has been active for a certain duration, irrespective of idleness.
idleTimeout: kills the session if it has been idle for the given duration. (The current TTL behaves in this manner.)

JIRA: https://issues.apache.org/jira/browse/LIVY-989

How was this patch tested?

Tested manually by creating interactive session with idle time and TTL.

[codecov-commenter]

Codecov Report

Merging #426 (9a1f31b) into master (8b2e29f) will decrease coverage by 36.78%.
The diff coverage is 48.57%.

@@              Coverage Diff              @@
##             master     #426       +/-   ##
=============================================
- Coverage     65.48%   28.71%   -36.78%     
+ Complexity      950      382      -568     
=============================================
  Files           103      103               
  Lines          6062     6084       +22     
  Branches        916      922        +6     
=============================================
- Hits           3970     1747     -2223     
- Misses         1541     3980     +2439     
+ Partials        551      357      -194     

Files	Coverage Δ
...va/org/apache/livy/client/common/HttpMessages.java	85.71% <100.00%> (-10.65%)	⬇️
.../main/scala/org/apache/livy/sessions/Session.scala	63.63% <100.00%> (-11.16%)	⬇️
...e/livy/server/interactive/InteractiveSession.scala	70.32% <85.71%> (+8.10%)	⬆️
.../server/interactive/CreateInteractiveRequest.scala	61.76% <33.33%> (-13.24%)	⬇️
...server/interactive/InteractiveSessionServlet.scala	55.69% <66.66%> (-3.66%)	⬇️
...cala/org/apache/livy/sessions/SessionManager.scala	58.92% <7.14%> (-23.43%)	⬇️

... and 80 files with indirect coverage changes

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[gyogal]

LGTM, thanks for your contribution!