Populate fields in snapshots created with stellar snapshot create

[leighmcculloch]

The stellar snapshot create command should populate the ttl fields based on the config settings of the network at that ledger.

Today those fields are 0:

  "timestamp": 0,
  "min_persistent_entry_ttl": 0,
  "min_temp_entry_ttl": 0,
  "max_entry_ttl": 0,

THe missing ttl fields especially cause problems when using the snapshot in contract tests because a max_entry_ttl of 0 means that storage cannot be extended as it normally would be able to.

[leighmcculloch]

Related to:

• Snapshot at least on testnet is quite empty #2207

[tupui]

Linked to #1740