Add ability to record a log of AsyncFileCached page accesses for some time window

[satherton]

In support of #1518, it would be nice to have an ability to log storage engine page accesses from a live cluster for use in evaluating different caching policies.

One easy to use form of this would be for the fdbcli "profile" command to support this via a subcommand. Each process will would accumulate page numbers accessed for the specified amount of time into memory and then write the collected data to a set of keys AFTER the profile period is complete (so as to not affect the workload during the collection).

The data could be logged to chunked key/value pairs with keys like <prefix>/filename/<chunkNumber>.