The wasm engine is moved from replica::database to the query_processor.
The wasm instance cache and compilation thread runner were already there,
but now they're also initialized in the query_processor constructor.
By moving the initialization to the constructor, we can now
be certain that all wasm-related objects (wasm instance cache,
compilation thread runner, and wasm engine, which was already
passed in the constructor) are initialized when we try to use
them because we have to use the query processor to access them
anyway.
The change is also motivated by the fact that we're planning
to take Wasm UDFs out of experimental, after which they should
stop getting special treatment.
Closes#13311
* github.com:scylladb/scylladb:
wasm: move wasm initialization to query_processor constructor
wasm: return wasm instance cache as a reference instead of a pointer
wasm: move wasm engine to query_processor
92318fdeae