Auto-reconnect in chainsync input plugin

[agaffney]

This should be optional but enabled but default. We should reconnect with a number of retries and throw an error on failure. The chain sync should be setup again from scratch, but with our last known chain point(s) as the intersect point

[agaffney]

Modifying snek to set a 1s keep-alive timeout for the chainsync input and running the following command will reproduce an "unexpected" disconnect fairly reliably after a few minutes. This is useful for testing.

./snek -filter-type foo -input-chainsync-bulk-mode=true -input-chainsync-intersect-tip=false -input-chainsync-intersect-point 39916796.e72579ff89dc9ed325b723a33624b596c08141c7bd573ecfff56a1f7229e4d09