CheckSchema: Icinga DB Schema update file in error

[oxzi]

CheckSchema: Icinga DB Schema update file in error

When Icinga DB detects a schema version mismatch, the expected schema
version number is shown in the error message. Unfortunately, the
internal schema version number is not reflected in the schema upgrade
filenames, matching the Icinga DB release version.

For example, starting Icinga DB after upgrading to version 1.4.0 without
applying the schema upgrade first, would complain that schema version 7
is expected for MySQL and schema version 5 is expected for PostgreSQL,
respectively. However, the shipped schema upgrade files do not reflect
these version numbers, but are named 1.4.0.sql, after the Icinga DB
release.

This output is confusing for end users. Thus, an internal mapping of
schema version numbers to Icinga DB releases ware created, allowing to
output the expected schema version number together with the Icinga DB
release version.

> Starting Icinga DB daemon (1.5.1-ga395295-dirty)
> Connecting to database at 'mysql://root@localhost:3306/icingadb'
> unexpected database schema version: v7 (expected v8), please apply the 1.5.2.sql schema upgrade file to your database after upgrading Icinga DB: https://icinga.com/docs/icinga-db/latest/doc/04-Upgrading/

In order to have a single point to update these versions, the schema
version numbers where moved to internal/version.go, where the Icinga DB
version is also being defined.

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Move Redis Schema Version to internal/version.go

After moving the database schema version number to the
internal/version.go file, the Redis schema version was moved there
as well. Now there is only one place left in the codebase to define
versions.

This reflects the release issue template, listing the
internal/version.go to be checked.

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CheckSchema: Loosen schema upgrade check

In f49fac079893d79d1ccbb40df67c1ae2e0705c2c, a strict schema upgrade
history was enforced. This might result in reports due to misaligned,
but fixed schema upgrades.

Now, this check was loosened to only enforce that all schema updates
between the lowest and latest schema version were applied. If they are
not in order, only a warning is being produced.

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Fixes #995.
Fixes #1068.