DB Fetch processors do not support Expression Language for Initial Max Value

As part of NIFI-2881, support for expression language was introduced for many/most fields in the DB Fetch processors (GenerateTableFetch, QueryDatabaseTable). However for the Initial Max Value properties (user-defined properties of the form initial.maxvalue.<column>), the documentation is inconsistent as to whether it supports EL, and if EL is present it is not evaluated.

The DB Fetch processors should be updated to fully support (and correctly document) the Initial Max Value capability. Note that the scope of EL evaluation may differ between GenerateTableFetch and QueryDatabaseTable, as the former accepts incoming flow files (via NIFI-2881) but the latter does not.