logd: prune maintain per-id watermark

Without this change LogBuffer::prune and LogBuffer::erase
contributes 16.7% and 1.79% respectively. With this change,
they contributes 3.06 and 2.33% respectively. Pruning is
performed roughly 1 in every 255 log entries, a periodic
tamer latency spike.

Bug: 23685592
Change-Id: I6ae1cf9f3559bca4cf448efe8bcb2b96a1914c54