model the Apache webserver concepts in Rust

[khklau]

The Apache module C API is a bit strange because it doesn't directly represent concepts like virtual host, worker MPM or event MPM. Where possible create an abstraction in Rust representing these concepts.

This might not be strictly necessary to get the tile module working, but should hopefully make the Rust code less confusing.

[khklau]

According to the book Writing Apache Modules with Perl and C:

There are different server records for each virtual host; your handlers will be passed the correct server record at runtime, either directly in the argument list (for example, for the child_init handler) or indirectly in the request_rec, where the record can be recovered from the server field.

This means that the only guaranteed way of identifying a server_rec instance would be via its pointer address (assuming Apache doesn't do something like a memcpy or memmove).