Allow dirty eviction per file as checkpoint progresses

At the moment the transaction ID for a checkpoint causes us to hold all of the updates required to service that transaction in memory (i.e: blocks eviction). In a workload that consists of many short transactions and regular checkpoints those updates can be unnecessary.

I'm planning to implement a scheme were:

• We track a checkpoint generation.
• Each file will reference it's current checkpoint generation. Once a checkpoint has finished in a particular file it will bump it's checkpoint generation.
• We track the oldest transaction ID excluding the checkpoint ID, and the checkpoint ID.

When checking whether a page can be evicted:

• If checkpoint generation is equal to the current checkpoint generation: Use oldest ID.
• If checkpoint generation is before the current checkpoint generation: Use older of oldest ID and checkpoint ID.

Use a similar scheme in the obsolete update check of

.

All other places in the code use the oldest ID as a visibility check. To implement the visibility changes I'll likely split 

__wt_txn_visible_all{{`}} into two variations - one as it currently exists and another that includes the checkpoint ID.