Support Reopen of Closed Iterations

[ax3l]

In reads, we often need to close iterations to avoid running out of resources.

Currently, there is no way to reopen a closed iteration.

Features

• In random access and in .readIterations mode, we need to be able to access an iteration again after we .close()d it.
• In random access mode, we need an option that closes the oldest iteration, e.g., after we opened N new ones. This could be done as the first thing once we open an iteration. Example: have only the last N accessed iterations open at a time (N being the configurable).

[franzpoeschel]

Will be closed by #1592