event_flatstore: fix rotation handling and add optional file headers

[nikbyte]

Summary
This PR contains several fixes and small improvements for event_flatstore.

Details
The main goal of this PR is to make file rotation more reliable and predictable, and to add a small usability improvement for generated log files.

It also fixes a few smaller issues in the module implementation.

Solution
The changes include:

• improved rotation handling;
• more predictable periodic rotation behavior;
• a new optional header parameter, which writes a first line to newly created log files;
• a few small fixes and cleanups in the code.

Compatibility
The change is backward-compatible.

The new header parameter is optional and disabled by default, so existing configurations will continue to work as before.

Closing issues
N/A