Add io_buffer_size to BackupEngineOptions

[archang19]

Summary

The RocksDB backup engine code currently derives the IO buffer size based on the following criteria:

• If specified, use the rate limiter burst size
• Otherwise, use the default size (5 MiB)

We want to be able to explicitly choose the IO size based on the storage backend. We want the new criteria to be:

• If specified, use the size in BackupEngineOptions
• If specified, use the rate limiter burst size
• Otherwise, use the default size (5 MiB)

This PR adds a new option called io_buffer_size to BackupEngineOptions and updates the logic used to set the buffer size.

Test Plan

I added a separate unit test and verified that we can either use the io_buffer_size, rate limiter burst size, or the default size.

I decided to use a TEST_SYNC_POINT_CALLBACK. I considered the alternative of updating the Read implementation of DummySequentialFile / CheckIOOptsSequentialFile to check the value of n. However, that would have considerably complicated the whole test code, and we also do not need to be checking for this in every single test case. I think the TEST_SYNC_POINT_CALLBACK turned out to be quite elegant.