Flush memtable when it reaches multiple megabytes

[Terkwood]

strategy

Run a GenServer which holds a Map tracking size per key and an int tracking total memtable size. Update total incrementally. Trigger flush when reaching limit. Clear GenServer on flush

Make sure you size both the key and the value

this ticket creates some debt

For #51 (binary SSTables) , we need to consider this new section of code