Nixpkgs: RFC: Move databases from /var/db to /var/lib for 17.09

[bachp]

Issue description

Currently some databases store their data in /var/db while others store it in /var/lib.
So for consistency reason I propose to move all databases to /var/lib. One reason for this is that this is the default on most Linux distros, and tools work out of the box. For example the MySQL tools expect the database to be in /var/lib/mysql if the path is not explicitly given.

As there are already breaking changes (https://github.com/NixOS/nixpkgs/pull/25931 and https://github.com/NixOS/nixpkgs/pull/25753) in the database area for state version 17.09 I think it might be a good occasion to clean this up.

What do the maintainers think about this?

Databases using /var/db:

• Firebird
• PostgreSQL => already moved in #25753
• InfluxDB
• Riak, Riak CS and Stanchion
• MongoDB
• OpenLDAP
• Graphite

Other tools in /var/db:

• uhub
• tahoe FS
• Firefox Sync server
• Name service caching daemon (nscd)
• Open vSwitch
• dhcpcd
• reaver-pws
• sudo

Databases already in /var/lib:

• HBase
• Redis
• 4store
• Virtuoso
• Cassandra
• CouchDB
• Neo4j
• Clickhouse
• MySQL & MariaDB (#25931)
• PostgreSQL (#25753)

[edolstra]

This sounds like a "much pain, little gain" change. It will require a lot of ad hoc, risky scripts to move data directories, and will make rollbacks impossible.

[bachp]

@edolstra I would not do automatic data migration. Just change the default if the state version is set to 17.09. This is how it is currently done for MySQL and PostgreSQL. See: #25931 and #25753

[dezgeg]

How about we create a compatibility symlink /var/lib/mysql -> /var/db/mysql?

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