Sometimes you want to have a default logger that is used
for all of your database connections; you can now set the
default logger using `gorm.SetDefaultLogger`.

When using `Preload` on a `many2many` association, the
`Query` callback chain was not being called. This made
it difficult to write a plugin that could reliably get
called regardless of how objects were being queried.

Now `handleManyToManyPreload` will call the `Query`
callback chain for each object that is retrieved by
following the association.

Since the data has already been read by the
`handleManyToManyPreload` method, a new scope setting
called `gorm:skip_queryCallback` is set to `true` before
calling the callbacks. Callbacks can check for the
presence of this setting if they should not be run; the
default `queryCallback` is an example of this case.

Fixes jinzhu/gorm#1621.

Adding / removing / replacing callbacks create logs; these logs
should be sent to `defaultLogger` rather than to the `log` system,
so that they can be properly processed.

Fixes #1231

The related go1.8 release notes: https://golang.org/doc/go1.8#database_sql

Saving a record with a non-Zero primary key previously would re-create
the record if the update affected zero rows. Now there will be an error
`ErrRecordNotFound` returned.