Add support for persistent SMTP connections.
Simplify tests, fix CS, remove `autoDisconnect` option.
Make `connect()` invoke authentication, make tests more explicit, add more tests.
Remove CacheHelper and related features.

CacheHelper is going to be split into a stanalone plugin for 3.x.
Removing this code will reduce confusion in the future about how to do
caching.
Use plugin prefixes when generating models in prefixes.

After a bit of discussion in #4026 it sounded baking associations that
default to using the plugin will result in less work instead of
defaulting to app models.
Don't generate `COMMENT ''` in MySQL when creating tables.

Refs #4031
Merge pull request #4049 from cakephp/3.0-schema-comment-blank

Don't generate `COMMENT ''` in MySQL when creating tables.
Merge pull request #4042 from cakephp/3.0-plugin-bake

Use plugin prefixes when generating models in prefixes.
3.0 - Fix issue where conditions that are valid global methods get treated as callables
Merge pull request #4054 from markstory/3.0-remove-cachehelper

RFC - Remove CacheHelper and related features.
Tests that callable is not a string
Merge pull request #4056 from dakota/patch-4

3.0 - Fix issue where conditions that are valid global methods get treated as callables
Ensure that the driver schema dialects use is connected.

Failing to connect the driver causes failures when identifiers are
quoted using cached schema objects.

Fixes #4031
Merge pull request #4062 from markstory/issue-4031

Ensure that the driver schema dialects use is connected.
Rename Cookies class to CookieCollection
Rename Associations class to AssociationCollection
Allow Cell::render() method to work with relative directories.

Refs #3945
Raise a cell specific error when views are missing.

The standard MissingView error page references controllers. By using
a more specific error message we can avoid referencing the controller
and only mention that the cell view is missing.
Add custom error page for missing cell errors.

Refs #3945
Empty the buffered content stack when rendering error pages.

When rendering an error page any buffered content should be discarded.
This fixes funny layout issues when cell errors are thrown, or when
missing element errors occur.