Add an EpochSyncTime table

[erikd]

This is mainly for measuring sync performance.

It would require a simple table like:

    EpochSyncTime
      no               Word64
      seconds          Double Maybe

Basiically we need an IORef or TMVar containing a Maybe UTCTime. On db-sync startup this mutable variable is set to Nothing. Then, on each LedgerNewEpoch event it retrieves the old value of the mutable variable and replaces it with the current time. If the old value is Nothing it writes Nothing to the seconds field of the table above. If old value is not Nothing the time in the mutable variable is subtracted from the current time and tihe difference in seconds written to the database.

One very minor gotcha that needs to be fixed first. Currently a LedgerNewEpoch event is generated erroniously on db-sync program start up. This would need to be suppressed for the above to give correct results.

[erikd]

It is probably a good idea to add a status field (Syncing vs Following) and maybe even an era field (eg Byron, Shelley, Allegra, etc).