the checksum object.

* Add helper methods to get a checksum object also from a string representation
of the checksum type.

* Always use the GetHexChecksum for retriving a consistent hex representation
of the binary checksum. Note the adler is interpreted as little-endian while
the rest are just a hex representation of the memory layout.

* When resetting the disk info in the local db set the file and block checksum
errors to false i.e. 0.