master: Handle concurrent metadata dump requests from Shadows

When Shadow fails to apply changelog entry, it requests metadata
dump from master. In case multiple Shadows request dump
in rapid succession - meaning master will be still in
process of handling first request while more requests arrive
master shall accumulate all requests and handle them together.

Additionally master performs metadata dump in background.

Change-Id: I2c38172be7e8803d108780d5b9b3c777ad38d99e