support for v2 txhashset (continuing to use v1 in mainnet and floonet)

[antiochp]

Resolves #3011.

• Cleanup TxKernelEntry wrapper (just store TxKernel directly in the kernel MMR data file)
• Add support for reading/opening kernel MMR with v2 protocol version
• Continue to support v1 kernel MMR -
  • all nodes on mainnet maintain v1 kernel MMR locally
  • fast sync will always provide v1 kernel MMR
  • but nodes now have the ability to read a v2 kernel MMR for future compatibility

This "future proofs" nodes so that once 2.1.0 is released and widely used we can think about letting nodes migrate to v2 kernel MMR.
Its not safe to do this today because nodes do not support v2 widely.
Fast sync can only really provide the same txhashset that is used locally as converting between v1/v2 cannot be done "on demand" or per fast sync request.

So this PR leaves all nodes continuing to use (and serve) v1 txhashset.
But with support for receiving a v2 txhashset in the future.

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tl;dr Nodes maintain a txhashset locally, receive a txhashset during fast sync and serve a txhashset to peers when they fast sync.
We want add support for receiving a v2 txhashset. But we want to continue to maintain and serve txhashset files in v1 for now, until v2 is widely supported on the network.