[refactor/resolver] lighter kong.lua

[thibaultcha]

It's been a while I was thinking kong.lua needed a refactor to be better organized. To be properly decoupled it needed a module to load the plugins for a given request, a module to be run manually at given points during a request, and kong.lua orchestrating both.

Changes:

• resolver is not plugin-like anymore.
• introduce kong.core: per-context handlers that need to be run before or after given Nginx contexts. It carries information about a request from one context to the next ones using ngx,ctx. Timers, plugins to execute, etc...
• kong.core.resolver is now ran as part of the before_access handler of kong.core.
• rename $backend_host and $backend_url to $upstream_host and $upstream_url.
• plugins can now access the ngx.ctx.upstream_url variable to modify it. kong.core will copy the value to $upstream_url just before the request is proxied,
• drop the redundant ngx.ctx.stop_phases variable.
• custom iterator to iterate through plugins in Nginx contexts. This makes kong.lua much lighter.
• fix: latency headers are now properly included in responses (that was not the case before).
• fix: fault-tolerant to interrupted contexts. This fixes Kong's interaction with binary nginx access modules #594. I was originally thinking I should check for ngx.status == ngx.OK, but this was not suited for plugin which still want to be executed when previous contexts failed.

Next (TODO):

• This is also a step towards removing dao and configuration from the globals. Instead, we want them to be allocated in kong.lua and passed along plugins through ngx.ctx (the job of kong.core).