coverage database support

[William Cohen]

I have been working on optimizing the performance of the coverage database code. 
  The removed column has been removed from the table, and the type has been 
changed from a string to an enums.

I also found that the collect_derivation_chain was just adding additional 
elements to the vector rather than creating a new vector from scratch each time. 
This was corrected reducing the runtime. A correction has been checked in CVS.

However, the killer test is testsuite/semok/twenty.stp. This creates a probe for 
every possible location. For the fc6 machine I am doing the experiments on. 
there about 523,000 probes. Each one of those probes would have an entry in the 
table. The coverage code appears to be inserting about 450 entries per MINUTE 
into the table. This would take about 38 hours to create coverage information 
for this one test and generate about 2G table (which is manageable size in sqlite).

Definitely, need to improve the speed of the insertion into the table. Would 
like to factor out  the file names and store, have the filenames as numbers in 
the main table, and have a separate table(s) store the map between number and 
file name.

-Will