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Return number of buffer entries when writing

For FIFO and RING buffers, return the current number of buffer
entries after adding the newly written element to the buffer.

For LIFO buffers the count cannot be safely determined in the
face of concurrency so just return 'true' (as before) that the
element was written to the buffer.
Set high water buffer fill and allow clearing
Add timestamps to ets_buffer entries
Merge pull request #13 from jaynel/issue-12-write-count

Return number of buffer entries on write to buffer