overview mechanisms

[Bo Lindbergh]

I myself said:
> Now that I can build ovdb again, I'll definitely run benchmark tests
> comparing ovdb and ovsqlite.

Quick and dirty test running "makehistory -O -x" on 544504 entries
with the cycbuffs and the overview data both stored on the same
external rotating rust device.

method	  real time  CPU time  disk space
========  =========  ========  ==========
ovdb  	     329.80     31.43 	     402M
ovsqlite     333.51   	54.73 	     200M


Development progress: feature complete and ready for the hard part:
documentation.


/Bo Lindbergh