unix socket support for state history endpoint 📦

[spoonincode]

Change Description

Oftentimes clients consuming from nodeos' state history endpoint fully trust the data received from this endpoint. It certainly doesn't have to be this way -- a client connected to the state history endpoint could perform light validation, for example. But oftentimes that is not the case; even rodeos fully trusts the state history endpoint it connects to.

This change adds a new option to state_history_plugin to expose its endpoint over a unix socket in addition or in lieu of the existing TCP endpoint. In some secure environments this allows more tight coupling of nodeos and rodeos (or other ship consumers) to enforce that the state history client always connects to the expected (trustworthy) nodeos.

The state-history-endpoint option has been changed to behave similar to http-server-address: if state-history-endpoint is configured as an empty string, the TCP endpoint is fully disabled.

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