Read epoch chunks one at a time #417

Problem

The epoch history and the investor's requests read every epoch, and every chunk of getLedgerEntries goes out at once. On a vault with many epochs, that burst can hit a public RPC's rate limit.

Expected outcome

Chunks are read one after another. Every epoch is still read, so no request is hidden.

Acceptance criteria

• readEpochs and readEpochRequests send one chunk at a time
• Test: with several chunks, at most one read is in flight