Separate DB and non-DB code to different structural sync bundles

Previously, code in org.simantics.structural.synchronization.client was
in packages that were dangerously named
org.simantics.structural.synchronization, which could easily escalate
into split packages between bundles with more of such sloppy naming.

This moves the whole org.simantics.structural.synchronization.base
package from .structural.synchronization.client to
.structural.synchronization which changes nothing from the bundle user's
point of view. Code that stays in .structural.synchronization.client is
moved into the org.simantics.structural.synchronization.client package
to keep things cleaners.

gitlab #206

Change-Id: I2366b6e7395750cc9b6649abd3e77ef1661d2d1f