make headers quasi-consistent (#660)

possible breakages:

* `FN_HEADER` on cold are no longer `s/-/_/` -- this is so that cold functions
can rebuild the headers as they were when they came in on the request (fdks,
specifically), there's no guarantee that a reversal `s/_/-/` is the original
header on the request.
* app and route config no longer `s/-/_/` -- it seemed really weird to rewrite
the users config vars on these. should just pass them exactly as is to env.
* headers no longer contain the environment vars (previously, base config; app
config, route config, `FN_PATH`, etc.), these are still available in the
environment.

this gets rid of a lot of the code around headers, specifically the stuff that
shoved everything into headers when constructing a call to begin with. now we
just store the headers separately and add a few things, like FN_CALL_ID to
them, and build a separate 'config' now to store on the call. I thought
'config' was more aptly named, 'env' was confusing, though now 'config' is
exactly what 'base_vars' was, which is only the things being put into the env.
we weren't storing this field in the db, this doesn't break unless there are
messages in a queue from another version, anyway, don't think we're there and
don't expect any breakage for anybody with field name changes.

this makes the configuration stuff pretty straight forward, there's just two
separate buckets of things, and cold just needs to mash them together into the
env, and otherwise hot containers just need to put 'config' in the env, and then
hot format can shove 'headers' in however they'd like. this seems better than
my last idea about making this easier but worse (RIP).

this means:

* headers no longer contain all vars, the set of base vars can only be found
in the environment.
* headers is only the headers from request + call_id, deadline, method, url
* for cold, we simply add the headers to the environment, prepending
`FN_HEADER_` to them, BUT NOT upper casing or `s/-/_/`
* fixes issue where async hot functions would end up with `Fn_header_`
prefixed headers
* removes idea of 'base' vars and 'env'. this was a strange concept. now we just have
'config' which was base vars, and headers, which was base_env+headers; i.e.
they are disjoint now.
* casing for all headers will lean to be `My-Header` style, which should help
with consistency. notable exceptions for cold only are FN_CALL_ID, FN_METHOD,
and FN_REQUEST_URL -- this is simply to avoid breakage, in either hot format
they appear as `Fn_call_id` still.
* removes FN_PARAM stuff
* updated doc with behavior

weird things left:

`Fn_call_id` e.g. isn't a correctly formatted http header, it should likely be
`Fn-Call-Id` but I wanted to live to fight another day on this one, it would
add some breakage.

examples to be posted of each format below

closes #329

api/server/runner.go

@@ -50,16 +50,6 @@ func (s *Server) handleFunctionCall2(c *gin.Context) error {
 	return s.serve(c, a, path.Clean(p))
 }
 
-// convert gin.Params to agent.Params to avoid introducing gin
-// dependency to agent
-func parseParams(params gin.Params) agent.Params {
-	out := make(agent.Params, 0, len(params))
-	for _, val := range params {
-		out = append(out, agent.Param{Key: val.Key, Value: val.Value})
-	}
-	return out
-}
-
 // TODO it would be nice if we could make this have nothing to do with the gin.Context but meh
 // TODO make async store an *http.Request? would be sexy until we have different api format...
 func (s *Server) serve(c *gin.Context, appName, path string) error {
@@ -67,7 +57,7 @@ func (s *Server) serve(c *gin.Context, appName, path string) error {
 	// strip params, etc.
 	call, err := s.agent.GetCall(
 		agent.WithWriter(c.Writer), // XXX (reed): order matters [for now]
-		agent.FromRequest(appName, path, c.Request, parseParams(c.Params)),
+		agent.FromRequest(appName, path, c.Request),
 	)
 	if err != nil {
 		return err