Support converting legacy storage structures on Webserver

Currently, it is very hard to change contents of storagestructs.thrift because we have several storages on ci-files01, testresults and coin master servers that require that all tasks are consistent. In order to change storage structures we either need to

 1) re-serialize all thrift files in all storage caches

 2) maintain code for handling legacy thrift specs

Option 1) has the benefit over 2) because we can keep the code base clean from handling different legacy formats because all thrift files are guaranteed to have consistent specs.

In order to make this more flexible we could implement a background service for Webservers that will do the conversion in as-needed-basis.