Fix: handful of resiliency fixes

[kantai]

This PR fixes a handful of issues that cropped up while running on testnet.

1. Ignoring L1 blocks before the first block height: this ensures that if a reorg occurs before the first block height, the node doesn't need to backtrack (potentially through blocks it hasn't received data for).
2. The neon run loop needs to increment the "target_burnchain_block", even without PoX reward cycles. To do this, we just use a constant value.
3. On restarting a paired stacks-node and subnet-node, the stacks-node may re-broadcast the last event it broadcasted. In such cases, the subnet-node should just drop the duplicate data.
4. The p2p inventory sync needs to send GetBlockInv messages even though the PoX reward cycle sync is broken in subnets (because there's no PoX cycles!)

[codecov]

Codecov Report

Merging #252 (7d373f0) into master (b32b7eb) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #252   +/-   ##
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  Coverage   93.47%   93.47%           
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  Files           6        6           
  Lines         337      337           
=======================================
  Hits          315      315           
  Misses         22       22           

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[obycode]

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