Handle future epoch tx appearing and disappearing when taking partition snapshot

[area363]

Currently, the snapshot takes care of future epoch tx that's fully established in the main chain.
For example, lets say that our first snapshot file is snapshot-1-1.zip. On the next snapshot schedule, if the current epoch is 1 and a future tx in epoch 3 exists, snapshot-1-3.zip will be created. If the future tx still remains in the next snapshot schedule, snapshot-1-3.zip will be created again. However, if for some reason the future tx disappears, snapshot-1-1.zip will be created and this will overwrite the original snapshot-1-1.zip. This will contaminate the snapshot file download order in the metadata and needs a fix.

[area363]

For a quick fix, snapshot will be adjusted to take 100 blocks before the preloaded chain tip (confirmation: 1).