Previously, we only sent the announcement when there were external
addresses. But it contains other information like the node alias,
therefore we always send it.
Since the proof-of-work can be expensive to compute everytime, we add
the ability to load the local node announcement from the file system.
We also change the proof-of-work parameters so that they are relaxed
when running in debug mode, instead of when running tests.
Finally, we remove the boolean validation of the PoW. Instead, we store
the work per-node, in the database, to be used in the future in case of
DoS attacks or network congestion.
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