Fix go get url

Fixes #904

OnStartup and OnShutdown callbacks now run as part of restarts, too.
The startup and shutdown directives only run their commands NOT as part
of restarts, as before. Some middleware that use OnStartup may need to
switch to OnFirstStartup and implement OnFinalShutdown to do any cleanup
as needed.

Minor refactor for rewrite.

* rewrite: fix new case
- added new test case and solution
- fix test case on rewrite_test

* replacer: change to table test

Fix missed if_op refactor. Ensure with tests.

* checkpoint

* Added RequestMatcher interface. Extract 'if' condition into a RequestMatcher.

* Added tests for IfMatcher

* Minor refactors

* Refactors

* Use if_op

* conform with new 0.9 beta function changes.

Also fixed a few typos and renamed caddyfile.ServerBlocks() to
caddyfile.Parse().

Refactoring to remove lint warnings

The search-and-replace was a little too aggressive and I accidentally
ran tests recursively in a subdirectory instead of repo's top folder.

- Server types no longer need to store their own contexts; they are
  stored on the caddy.Instance, which means each context will be
  properly GC'ed when the instance is stopped. Server types should use
  type assertions to convert from caddy.Context to their concrete
  context type when they need to use it.
- Pass the entire context into httpserver.GetConfig instead of only the
  Key field.
- caddy.NewTestController now requires a server type string so it can
  create a controller with the proper concrete context associated with
  that server type.

Tests still need more attention so that we can test the proper creation
of startup functions, etc.

* Add timeout to http get on health_check

* Add new test and up the timeout

* Tests for change to default timeout

* Only call http client once and make options more inline with current caddy directives

Refactor and clean up policy code

This commit shouldn't change any behavior. It is simply a cleanup of
the different proxy policies. It also adds some comments explaining the
sampling method used, since on first inspection it might not appear to
be a uniformly random selection.

Fix rewrite bug with URL query + test case (#884)

Add minify directive

* Balance round robin evenly when some hosts are down

Before, when load balancing across multiple hosts, if a host went down
then the next host in line would be sent a double share of requests.
This is because the round robin counter was only incremented once per
request, regardless of the health of the selection. If current
selection was unhealthy then the policy would advance to the next host,
but this would not be reflected in the policy counter. To fix this, the
counter is now incremented for every attempted host.

This commit adds a test case that identifies the issue, and a fix.

* Make robin counter private

* Use a mutex to sync round robin selection

* Add reverse_proxy preset

* Update to 'transparent' preset instead of 'reverse_proxy'

Matches the new upstream function signature and fixes previously broken
behavior; new solver code confirmed to work during restarts

Also we change the scheme of the site's address if TLS is enabled and
no other scheme is explicitly set; this makes it appear as "https" when
we print it; otherwise it would show "http" when TLS is turned on
implicitly, and that is confusing/incorrect.

Fix restart on USR1 not loading new Caddyfile

Add a {request} placeholder to the replacer.

Closes #858.