composite_formatter and eval_formatter APIs deprecated

[chaley]

The template formatter instance ebooks.metadata.book.base.composite_formatter has been deprecated. It is a static instance of SafeFormat. SafeFormat is not thread safe, making using the static instance problematic. The composite_formatter declaration will be removed after a release or two.

If you use composite_formatter, then replace your code that looks like

composite_formatter.safe_format(...)

with

from ebooks.metadata.book.base import SafeFormat
...
SafeFormat().safe_format(...)

You can make a local instance of SafeFormat if you wish, as long as you can guarantee that the local instance will never be used simultaneously by multiple threads.

utils.formatter.eval_formatter has the same problem, and should be fixed in a similar way.