MongoDB doc size can exceed limit for veto_intervals

[cfuselli]

Describe the bug
If too many veto intervals are fired, the size of veto_intervals can become too big (>16MB) to be uploaded in the online monitor frontend (see here).

This happened for example for run 051642 (due to intense HEV). The outcome is that the entire processing for the run fails.

The error states:

2023-04-03 13:05:01.133224 | INFO     | process_run (L1419)  | Still processing run 051642. PID:18185
Exception in thread save_1:veto_intervals:
Traceback (most recent call last):
.
.
.
    raise DocumentTooLarge(
pymongo.errors.DocumentTooLarge: BSON document too large (24848171 bytes) - the connected server supports BSON document sizes up to 16793598 bytes.

Maybe need to include this into online_monitor data types with some kind of size check.