Instead of taking snapshot every N blocks, this CR
changes it to taking snapshot every N bytes.

This also sets default SnapshotInterval to 100MB, if
it's unset. Otherwise data in memory is never compacted
till OOM.

Meanwhile, DefaultSnapshotCatchUpEntries is shrunk so
it does not take too much space to preserve extra entries
every time a snapshot is taken. Slow nodes are catching up
using blockpuller, which is also efficient.

Change-Id: I79cfeb8652fcbafdeb5793bf4f06267b95a858d6
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>