InMemoryLog should use a growing mechanism similar to FilePositionCache

[franz1981]

FilePositionCache allow accumulating entries while appending fixed sized chunks/pages saving expensive memory copies while enlarging capacity: InMemoryLog could make use of a similar mechanism for its entries.

Given that InMemoryLog is not for production usage this is more a wish then an actual requirement, although it's pretty handy to use it to benchmark the whole raft protocol machinery without I/O

[belaban]

I'm all for replacing InMemoryLog with ArrayRingBuffer, as the former only needs to keep the uncommitted entries (in my experience with CounterPerf a few 100 entries) in memory. InMemoryLog should not be taken as a prod-ready impl; it was written with unit tests in mind (where perf doesn't matter)...