server: emit HTTP headers to disable caching
server: emit HTTP headers to disable caching

This week I'm using an ISP that transparently proxies and caches HTTP
results. This made for some very confusing experiences running
dist-test, since the API calls to fetch job status were returning cached
results indicating none of the tasks were completing.

This adds the appropriate server-side headers to disable caching. I
haven't verified in the real environment that this disables my ISP's
cache, but I did verify using 'curl -I' that the headers are present on
/job_status requests against a local install.

cloudera/dist_test,xiao-chen/dist_test,umbrant/dist_test,xiao-chen/dist_test,cloudera/dist_test,umbrant/dist_test,xiao-chen/dist_test,cloudera/dist_test,umbrant/dist_test,umbrant/dist_test,cloudera/dist_test,xiao-chen/dist_test