Investigate potential file-handle leak

Running 2.x on macOS using default `ulimit` settings can quickly lead to:
```
run: open server: open tsdb store: fcntl: too many open files
```
The "fix" we've been using internally is to bump the open-file limit to a big number in our shell profiles, i.e.
```
ulimit -n 10000
```
We need to investigate why this is needed, and if it's something expected vs. a leak.

Start by reproducing the `open server: open tsdb store: fcntl: too many open files` error on macOS with default `ulimit` settings, then compare it with the reported `ulimit -n 10000` workaround. Trace the server and TSDB store file handling to determine whether the behavior is expected or indicates a leak; done means documenting the cause and an appropriate resolution.

Written by the indexing model from the issue text.