API: Add support for selected HTTP precondition headers

API: Add support for selected HTTP precondition headers

Specifically, GET requests can now return ETag and Last-Modified
headers, and If-None-Match and If-Modified-Since headers on such GET
requests will be honored. This doesn't change any API modules to
actually return these values, it just provides the infrastructure.

For reasoning on why only GET requests and why only these two of the
five precondition headers defined by RFC 7232, see the doc comment on
ApiMain::checkConditionalRequestHeaders().

Change-Id: Ia18874c9360fcffdad323b341ca867ba773788fd