CWhisper: support (limited) backfilling/history rewrite

[bom-d-van]

As the tool and I aged, a realization of a possibility of supporting users to overwrite existing timestamps in cwhisper has come to my mind.

In cwhisper, there is a simple index data structure of each archive, which is basically an array of start and end timestamps. As long as users are not sending timestamp to the current block that isn't smaller/older than the start of the previous block, it's possible for cwhisper to correctly return the last written data.

Of course this means in ReadFromBlock, cwhipser needs to parse the whole block when decoding data. This might slows down the render api for some queries (benchmarks needed still), but should be a good compromise.