Fix url limit param & Gerrit query for relative age searching

When generating the Gerrit query use absolute before/after searching if server supports it
AfterSearch: Use "age" as the op name if we are generating a relative Gerrit query
GerritURL: Serialise limit field
Config: Fix database cursor not closed
GerritService: Manage currently running SyncProcessors in HashMap

Change-Id: Ic5daa5387f2e6fcf06d2df5e2fbc56a93391fd25