When enabled, lines like "# interval-start 1234567890123456" and "#
interval-end 1234567890123456" will be added to the data output.  Note
that there may be an "interval-end" timestamp followed by an identical
"interval-start" timestamp, if the response at the nilmdb level was
split up into multiple chunks.

In general, assume contiguous data if previous_interval_end ==
new_interval_start.

Added new flag "-R" to command line to perform an automatic removal.
This should be the last of the ways in which a single command could
block the nilmdb thread for a long time.

Server class will retry as needed, as with stream_extract and
stream_intervals.

Instead, always take the subset, which creates a new interval.
Also adds a small optimization by moving the 'if orig' check outside the
loop.

Clients might need to to Interval math too, so move a simple Interval
class and start putting helpers in there.

This lets us use simple indexing to get timestamps from the table,
which allows us to use 'bisect' directly without needing a proxy class.

Implemented in command line, client, server, nilmdb, bulkdata

Previously, we ignored problems when sending intermediate blocks,
since getting more data might make the next attempt succeed.
But in practice, malformed data would just build up, causing
problems.  Raise an exception if there's too much data remaining
after trying to send an intermediate block.

strtod() and friends will happily swallow newlines, so we need to skip
over spaces and look for that sort of thing manually.

The bad line is printed out on a new line, and a third line
with a ^ to indicate the position of the error.

For rows and seconds only.  Extents still give None if they don't
exist.

Can show the set-difference between the interval ranges in two
streams.

Enable the following pragmas: synchronous=NORMAL, journal_mode=WAL.
This offers a significant speedup to INSERT times compared to
synchronous=FULL, and is roughly the same as synchronous=OFF
but should be a bit safer.

Also changes the HTTP parameter from "extent" to "extended",
and the commandline parameter from "extent" to "ext".

"import nilmdb" doesn't do much; "import nilmdb.client" or "import
nilmdb.server" is now required.

Fixes various corner cases and other bugs regarding lines with
comments, having data but no endpoints, etc.

Things are now block-focused, rather than line-focused.  This should
give a pretty big speedup to inserting client data, especially when
inserting preformatted data.

Still needs work.