Add debug info to elp

Summary:
Users reported instances of ELP being repeatedly slow (eg T196163271). To ease troubleshooting, we can add debug-info when building elp. This allows us to dump a backtrace of all threads of a problematic ELP instance by running:

```
ps aux | grep "elp server" | grep -v grep | awk '{print $2}' | xargs -I{} lldb-stable -p {} --batch -o 'thread backtrace all' > /tmp/debug.txt
```

Using the "line-tables-only" option ensures we only add the minimum amount of debug info we need, including filename/line number info, but no variable or function parameter info.

With the above changes the executable jumps from 41Mb to a whopping 141Mb, but I think we can live with it, at least until we find the root-cause of the slowness.

Reviewed By: alanz

Differential Revision: D61378510

fbshipit-source-id: bdbd5bcc738cf709452643d42e3f446c20290a13

.cargo/config.toml

@@ -6,7 +6,7 @@ xtask = "run --package xtask --"
 
 [profile.release]
 lto = "thin"
-strip = true
+debug = "line-tables-only"
 
 # Workaround to easily locate workspace root
 # See https://github.com/rust-lang/cargo/issues/3946