Logger: remove log record str conversion

[heitorlessa]

Logger include some default logging expr, and that requires string interpolation using %.

That means not all types that are JSON serialized are supported such as bool, for example.

As part of last GA feature (docs #37), we should capture and do string interpolation for reserved logging expressions only e.g. %(asctime)s

[heitorlessa]

Other areas that need updating:

• Lambda context model - memory_size field
• Lambda cold_start key