Revert "Harden disk-based message cache and implement size limit"

Unfortunately setting a size limit for MapDB would lead to a non-functional
message cache if the limit was hit. Additionally, online compaction (e. g.
while the server was running and still processing messages) didn't work at
all so that the system couldn't recover from a once filled cache at all.

This reverts commit 3404896511f3fb437b879daa62501636f3a5d532, refs #731.