rts/eventlog: Ensure that all capability buffers are flushed

The previous approach performed the flush in yieldCapability. However,
as pointed out in #19435, this is wrong as it idle capabilities will not
go through this codepath.

The fix is simple: undo the optimisation, flushing in `flushEventLog` by
calling `flushAllCapsEventsBufs` after acquiring all capabilities.

Fixes #19435.