Use write bounds to denote non-continguous writes

[blt]

Previously hopper suffered fragmentation problems in its array of memories. In particular, it was possible for a receiver to lose writes and re-detect them at a later time in an out of order fashion. The basic access pattern of hopper has always assumed a continually growing "index" in memory, which was disk. With the introduction of the actual in-memory buffers it was possible for either the sender or the receiver to backtrack and then skip a chunk of memory to avoid reading in something from a later write.

The solution is to store the bounds of writes. A receiver keeps a rolling ID of its reads which the senders use to note when a new bound is needed, owing, potentially, to the current bound having been invalidated. So long as the sender captured receiver id remains the same the current write boundaries grow.

Resolves #4

Signed-off-by: Brian L. Troutwine blt@postmates.com

[codecov-io]

Current coverage is 93.54% (diff: 100%)

Merging #5 into master will increase coverage by 0.36%

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