Also introduce caddy.OnProcessExit which is a list of functions that
run before exiting the process cleanly; these do not count as shutdown
callbacks, so they do not return errors and must execute quickly.
We renamed caddytls.ErrStorageNotFound to caddytls.ErrNotExist to more
closely mirror the os package. We changed it to an interface wrapper
so that the custom error message can be preserved. Returning only "data
not found" was useless in debugging because we couldn't know the
concrete value of the error (like what it was trying to load).
Users can do a type assertion to determine if the error value is a "not
found" error instead of doing an equality check.
* Initial concept for pluggable storage (sans tests and docs)
* Add TLS storage docs, test harness, and minor clean up from code review
* Fix issue with caddymain's temporary moveStorage
* Formatting improvement on struct array literal by removing struct name
* Pluggable storage changes:
* Change storage interface to persist all site or user data in one call
* Add lock/unlock calls for renewal and cert obtaining
* Key fields on composite literals
