Fix HazelcastHttpSession's setAttribute() with null value

HttpSession.setAttribute() should work with null value the same as removeAttribute(). Changed from throwing an exception to delegate call to removeAttribute()
Lots of minor cleanup
More work
Supression fixes
Resolved findbugs issue
Removed unwanted findbugs supression
Resolved merge conflicts
Fixed NPE
Merge pull request #2830 from pveentjer/checkstyle/3.x/invocation

Checkstyle/3.x/invocation
Merge pull request #2627 from akerekes/master

Fix HazelcastHttpSession's setAttribute() with null value