Multi-module support

[gzurl]

Currently, only one Wasm module with a unique configuration can be loaded at a time.

At first glance, we might want to consider two different scenarios:

1. Different Wasm modules on the same Apache instance, each of them attached to a different route.
2. One Wasm module serving different routes, but with different configurations.

In this issue, we will focus on 1). See #7 for the scenario 2).

Different Wasm modules at the same time involve, at least:

• Data structures storing httpd.conf mod_wasm directives need to be duplicated (once per module).
• Data structures for WASI context need to be duplicated too (ie: stdout buffer)
• Incoming HTTP requests need to be routed to the right Wasm module.

A first goal for this feature could be to serve PHP and Python Wasm modules by only one Apache instance.