[mapserver-dev] RFC109: runtime substitution refactoring

[thomas bonfort]

Devs, please have a look at rfc 109.

tl;dr for probably everyone except SteveL:

- speeds up all mapserv operations, especially when runtime
substitutions are not in use.
- allows runtime substitutions in metadata entries
- has a theoretical backward incompatibility that should not be an
issue in practice

best regards,
thomas