fix(relayer/cursor): dont overwrite existing

[corverroos]

Don't overwrite existing cursors. Just check if offsets are identical.

This fixes the following problem:

• Cursor X is inserted into the DB
• Cursor X is confirmed (so it is the only cursor for this streamer).
• Worker resets, starts streaming from X (inclusive)
• Cursor X is overwritten (resetting it to unconfirmed).
• Worker resets (before cursor can be confirmed). In this case, there are no confirmed cursors.
• So worker starts streaming from MUCH lower height (overloading archive node).
• MUCH lower cursors become new confirmed height.

We could also start streaming from X+1 instead, since confirmed cursors are fully finalized, so no need to reprocess to handle partial submissions.

issue: none