Make sure that case-insensitive column names are unique

[FabianHertwig]

This closes #21.

When there are metadata entries with the same case insensitive string, then there is an error when trying to create a new column for that metadata entry in the database table, because a column with that case insensitive name already exists.

 <Record type="HKQuantityTypeIdentifierDietaryCholesterol" sourceName="MyFitnessPal" sourceVersion="35120" unit="mg" creationDate="2021-07-04 20:55:27 +0200" startDate="2021-07-04 20:55:00 +0200" endDate="2021-07-04 20:55:00 +0200" value="124">
  <MetadataEntry key="meal" value="Dinner"/>
  <MetadataEntry key="Meal" value="Dinner"/>
 </Record>

The code added in this PR checks if a key already exists in a record and if so adds a number at its end. The resulting column names look like the example below then. Interestingly, the column names viewed with Datasette are not case insensitive.

startDate, endDate, value, unit, sourceName, sourceVersion, creationDate, metadata_meal, metadata_Meal_2, metadata_Mahlzeit

[FabianHertwig]

Did not see that #13 fixes the same issue in a similar way. You can decide which one to merge ;)