SourceLayout: format namespace for custom tag-based formats

SourceLayout: format namespace for custom tag-based formats

Part 1 of enabling non-logging components to support custom formats in strings

Shuffle the log custom format code into its own library separate from the
logging functionality.

One minor logic change removing redundant LogFileEnabled flag.

TODO:
 - use MemBuf instead or as well as StoreEntry as the output buffer
 - separate from AccessLogEntry confusion
 - upgrade deny_info URL generation format
 - upgrade external_acl_type format
 - add custom helper formats