Millisecond-Resolution Close Times

[SirTyson]

We would like to increase the resolution of closeTime to MS resolution. Currently, closeTime is in whole seconds, making sub-second ledgers and intra-second block times (i.e. 2.5 seconds) impossible at the protocol level. This change will allow the protocol to support lower block times. This change does not inherently lower block times, but opens the door for future upgrade via network config settings.

[JFWooten4]

Is the intent to change the unit of TimePoint globally from seconds to milliseconds, or only the interpretation of StellarValue.closeTime

And there's also a historical boundary problem to solve since ledger N closeTime = 1777000000 has the same valid uint64 as ledger N+1 after the upgrade: closeTime = 1777000001000

I don't think we'll ever be able to push it below 3.2 (spitballing) because it's disingenuous to give less than 2.5 seconds for network propagation.