Extend Rucio to generic checksum column

[bari12]

Motivation

Right now Rucio only supports adler32 and md5 checksums via two specific columns for these checksums. This creates problems if a community has different checksums (e.g. CMS historically used crc checksums) and wants to store them. On the dev meeting on 2019-03-28 it was discussed to extend Rucio with a generic checksum column of string type. Multiple checksums can be added (comma-separated) to this column, always specifying the checksum type. E.g. md5:1234,adler32:5678,crc:123

We would have to specify a strategy in migrating to this new checksum column.

[mlassnig]

Alternatively, a json column? I don't like string parsing too much

[bziemons]

is #3538 related to this?

[bari12]

Not really. We do provide already adler32 and md5 columns in the database. #3538 is just about an issue that at some parts of the code adler32 is falsly required.