Simplify use in cases where some levels support async flush and some do not

[adammoody]

Currently, the user must be careful to configure SCR_FLUSH to line up with their checkpoint descriptors if they want to use async flush on a multi-cache system, where only some levels support async functionality. This is cumbersome for users to keep track of. Let's make this easier.

We could either just fall back to sync flush in the case where the output lands in a cache location that only has sync or we could always select a cache location that supports async flush, even if the checkpoint id would normally write it to some other level.

[adammoody]

We can now support different flush mechanisms within a run using the TYPE option in the STORE descriptor. A flush will use the type specified by the user.