Log Format token namespace upgrade

Log Format token namespace upgrade

This updates the format parser and storage objects in the Format::
namespace and separates some into separate files.

Add a registration API so components can register themselves an array
of tokens in a namespace. Registering the arbitrary namespace "example"
with some tokens ("a","b") will cause the parser to accept those tokens
in a logging format like so: "%example::a %example::b".

Future work:
 - use runners registry instead of Init() function
 - convert the error pages to use format for the page body macros
 - convert the %ssl_* tokens in src/ssl/* to use format and "ssl::"
 - convert external_acl_type to use formats for its helper input string.