Hot-swapping Engines

[gburgett]

One of the key features of XFlat is that engines are hot-swapped as the tables grow. We need to define the procedure for this and create the implementation as part of the Database.

This is a bit tricky as there can easily be concurrent reads and writes while we are hot-swapping. We want to be read-responsive as a priority, and write-responsive as much as possible. I've written down thoughts in thoughts.md