spike: cache matchTree

[stefanhengl]

Webserver creates a new matchTree for every shard it searches. The structure of the matchTree, however, only depends on the query. Some trees, like substrMatchTree, call 'iterateNgramsand thus depend onindexData`. We should timebox this spike and see whether

1. matchTree construction shows up in webserver's CPU profile
2. there is a good way to hydrate a cached matchTree with indexData, instead of creating it from scratch every time.