[clangd][index-sever] Limit results in repsonse

This is to prevent server from being DOS'd by possible malicious
parties issuing requests that can yield huge responses.

One possible drawback is on rename workflow. As it really requests all
occurences, but it has an internal limit on 50 files currently.
We are putting the limit on 10000 elements per response So for rename to regress
one should have 10k refs to a symbol in less than 50 files. This seems unlikely
and we fix it if there are complaints by giving up on the response based on the
number of files covered instead.

Differential Revision: https://reviews.llvm.org/D101914