Gracefully handle decoding more summaries than statically known eras

[mrBliss]

Old wallets (or other clients) that are only aware of Byron and Shelley should
be able to communicate with nodes until the hard fork to Allegra actually takes
place. At that moment, they will stop functioning as they don't know about
Allegra. The NodeToClient versioning mechanism takes care of this.

However, we have noticed that when the update proposal for Allegra becomes
stable, the old clients' Summary decoder starts to fail with:

Summary: expected between 1 and 2 eras but got 3

This is because after the update proposal for Allegra has become stable, the
node (which is compatible with Allegra) will extend its Summary with an
EraSummary of the Allegra era. The Summary decoder of the old client does
not support more EraSummarys (3) than eras it is aware of (2) and fails. This
is /before/ the hard fork has actually happened. While the window between this
and the actual hard fork, at which point the old client would stop functioning
anyway, is small, it would still be nicer if this were handled gracefully.

So instead of failing when we receive more EraSummarys than eras we statically
are aware of, we ignore them. For example, a client that thinks Shelley is the
final era will ignore the EraSummary of Allegra (and Mary) and will keep
acting as if Shelley is the final era. Of course, when the hard fork to the next
era actually happens, the client will stop functioning as before.

Note that this fix is too late for the upcoming Allegra hard fork: only clients
that haven't been upgraded to the last release will run into this problem.
Upgrading them to the next release that includes this fix already makes the
problem go away, as they will gain support for Allegra (and Mary).

This fix will thus only help the next time, i.e., when clients are running a
version that supports Allegra and Mary (and includes this fix) but not the era
after that, i.e., Alonzo, and the update proposal to Alonzo has become stable.

[edsko]

Approved, under protest :)

[mrBliss]

bors merge

[iohk-bors]

Build failed:

• ci/hydra:Cardano:ouroboros-network:required

[mrBliss]

bors merge

[iohk-bors]

Build succeeded:

• buildkite/ouroboros-network
• ci/hydra:Cardano:ouroboros-network:required

[dcoutts]

Nice.