Refactor and improve usage of the context package

[tomcat-bit]

We must refactor and improve the usage of the context to make downstream HTTP requests more resilient and fault-tolerant. Client request handlers must use sane timeout values and context deadlines. Any clean-up procedures in the handlers must run, also in the case of failing to handle the requests. Proper error values need to be returned, if any. The policy store, directory server and node each use their own HTTP server. See http.go in each package.