Fast sync mode support

[s1na]

Currently XDC supports 2 sync modes: full sync and full sync with archive persistence (or archive mode). They both require processing every block since genesis. This obviously takes a very long time. To avoid this the foundation has been providing static snapshots of the datadir regularly which can be downloaded and used as a baseline.

This process complicates the running of a node. Further it requires downloading a 400Gb file in one go. And it is generally not as trustless. Go-ethereum used to have a fast sync mode which will download state data on the fly from other peers and sync to a recent block. It will be interesting to test whether this sync mode can work for XDC (which has way lower block times). Because otherwise the newer snap sync requires significant changes to the database layout and networking primitives, making it a more time-intensive task.

This is the initial PR that introduced the feature however it is quite old and has been modified over time: ethereum#1889