isisd: delay lsp regeneration while events are still coming in

isisd: delay lsp regeneration while events are still coming in

When there is a stream of events coming in, where IS-IS learns
about a lot of updates, IS-IS would regenerate its LSPs before
the updates have been processed completely.

This causes suboptimal convergence because the intermediate state
will be flooded. Only after the configured `lsp_gen_interval`, a
new update with the correct and final state will be generated.

Resolve this by holding off LSP generation while there are still
events coming in.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>