Set timestamp filter lower bound to current time

[pm47]

We previously assumed that the gossip_timestamp_filter only applied to
future messages, so we set first_timestamp to 0 in order to have a
pass-all filter.

But BOLT 7 actually says that the remote peer:

SHOULD send all gossip messages whose timestamp is greater or equal to
first_timestamp, and less than first_timestamp plus timestamp_range

Which means that the way our filter was set, the remote peer would dump
the entire routing table on us.

By setting first_timestamp to the current time, we achieve what we
want. The synchronization of older messages is done by sending a
query_channel_range and then one or several query_short_channel_ids.