set: add timeout support for sets

set: add timeout support for sets

Timeout support can be enabled in one of two ways:

1. Using a default timeout value:

set test {
	type ipv4_addr;
	timeout 1h;
}

2. Using the timeout flag without a default:

set test {
	type ipv4_addr;
	flags timeout;
}

Optionally a garbage collection interval can be specified using

gc-interval <interval>;

Signed-off-by: Patrick McHardy <kaber@trash.net>