Classes deriving from ExperimentData usually contain the
experiment-specific Protobuf message, which needs to be properly
destroyed. This is particularly a problem in the generic
DatabaseCampaign, as it never downcasts ExperimentData objects
retrieved from JobServer::getDone(). As the embedded
DatabaseCampaignMessage (usually named "fsppilot") is allocated on the
heap (this happens in the campaign's cb_send_pilot() function, asking
for a mutable_fsppilot()), the lack of a virtual destructor in
ExperimentData led to a memory leak, rendering the campaign server
inoperable after handling ~1E7 messages (with a 4GiB / 32-bit process
memory limit).
Change-Id: I4cb8a26d5a702e03189c4aae340051ce62a9c9ce
5ccc6e3525