Updates to connection strings are never applied to existing gatherers

[0xgouda]

Describe the bug
Any update to a source's connection string (e.g., param change, pointing to a different database, etc.) is never applied to already started gatherers

To Reproduce
Steps to reproduce the behavior:

1. Update reaper.go:506 to show the conn str in the logs l.WithField("cache", fromCache).WithField("rows", len(data)).WithField("conn_str", md.ConnStr).Info("measurements fetched")
2. Run pgwatch with a single source and some metrics
3. Update the conn str of the used source
4. Observe that the old gatherers will keep using the old conn str

Expected behavior
Should we allow updating the conn str of a source? If we should, this is a bug; if we shouldn't, we should disallow it from the web UI at least and mention it in the documentation.

Screenshots
two metrics for a single source fetching from 2 different databases because one of them is still using the old conn str:

2025-12-18 16:40:37.626 [INFO] [source:test] [metric:db_size] [rows:1] [cache:false] [conn_str:postgresql://postgres:postgres@localhost:5437/postgres] measurements fetched
2025-12-18 16:40:42.603 [INFO] [source:test] [metric:wal] [rows:1] [cache:true] [conn_str:postgresql://postgres:postgres@localhost:5437/pgwatch_metrics] measurements fetched

[0xgouda]

@pashagolub, what do you think about this?

[pashagolub]

if this is true, then it's certainly a bug

[pashagolub]

any change to the source definition should

1. stop the running gather
2. sync the info
3. start a gather with new config