Don't add inbound peers to the Routing Table till we've finished bootstrapping

[aarshkshah1992]

We should NOT add remote peers who make inbound connections to us to the Routing Table till we've refreshed/bootstrapped our Routing Table ATLEAST once upon startup.
This will prevent malicious peers from connecting to us en-masse upon startup and hijacking our Routing Table.

Also, if we ever detect an empty Routing Table, we should again disallow inbound peers till we've bootstrapped the DHT.

ping @Stebalien @aschmahmann.

[aschmahmann]

Shouldn't hurt, but I'm not sure how much this will really help us especially once we land the persistent routing table snapshotting + restoring.

The case of "if we ever detect an empty Routing Table" is also sort of flaky. what if my internet connection dies, but I'm still attached to a single peer on my local network? Over time if they are in DHT auto mode perhaps they'll switch into client mode and so my routing table will empty, but otherwise (e.g. multiple infrastructure servers that have been configured to server mode) they'll allow inbound connections as soon as they're online again.