Use a prefix of field:value for index length

Otherwise we cannot have long field values.

To accomplish this we inspect the table metadata brought in
from the sqlalchemy3 plugin and change the index data to
support a mysql length. The value is 191 because that is 767/4.

767 is the maximum number of bytes wide an index may be. 4 is the
max number of bytes per character in utf8mb4.
release 3.0.12