databases: update hikari minimum connection configuration

Commit 004d4d0c updated the version of Hikari to v1.3.5.  This new version of Hikari removed
support for maintaining a minimum number of database connections.  Attempts to configure this
minimum number of connections result in a warning being logged:

    The minimumPoolSize property has been retired, remove it from your pool configuration to avoid
    this warning.

Hikari now supports maintaining a minimum number of idle connections.  This behaviour is
substantially different from the previous minimum number of connections, therefore this patch
forbids setting the old *.db.connections.min properties and new *.db.connections.idle properties
are introduced.

This patch also adds missing annotations to help sites migrate their database pool configuration
from BoneCP to Hikari.

Target: master
Request: 2.9
Patch: http://rb.dcache.org/r/6933/
Acked-by: Gerd Behrmann
Acked-by: Tigran Mkrtchyan
Merge pull request #502 from paulmillar/fix/2.9/hikari-warning

databases: update hikari minimum connection configuration