Investigate possibility to use read node of database for mod-oaim-pmh

Currently to read data from database for harvesting mod-oai-pmh uses external dependency vertx-jooq-classic-reactive v.6.5.4 and RMB is used only to add data in the tables.

As example org.folio.oaipmh.processors.MarcWithHoldingsRequestHelper which has logic for harvesting uses two above options.

The same time in mod-oai-pmh vertx-jooq-classic-reactive is also used for some crud repositories with mod-oai-pmh entities.

According documentation https://github.com/folio-org/raml-module-builder#read-and-write-database-instances-setup the read node has such configuration:

The environment variable DB_HOST_READER sets the read database host's URI, if there is a reader instance.

The environment variable DB_PORT_READER sets the read database host's port, if there is a reader instance. It is to be used when DB_HOST_READER is set.

So to use read node of database there are two options:

1) Create separate connections for read database node : 

It will be needed to support usage  DB_HOST_READER and  DB_PORT_READER all time. The read node may not exist. If database cluster will not have a reader node it will be necessary to use values of a write node. 

In the code it will be needed to divide write and read operations to use proper a write or read connection.

2) Moving to RMB approach.  The benefits from it :

1. Most of the read-only code in RMB has already been updated to communicate to the Read node directly.
2. Samples at folio repositories  to create repositories for module's entities based on RMB.
3. The standard deployment configuration for the RMB modules. 

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