The <ArtifactMap> element configures the plugin used for the short term storage of XML messages bound to SAML artifacts. This is a very rarely used feature involving support for sending messages outbound via the HTTP-Artifact binding in SAML 2.0, which is virtually unheard of. Very few, if any, IdPs would support it. So, no, you probably don't need it.
The only supplied implementation is a wrapper around a <StorageService> plugin. It also includes an optimization for in-memory use that reduces the XML round tripping required, so binding it to an in-memory StorageService is not strictly necessary if the intent is to leave the data stored in memory.
When omitted, the optimized in-memory version with default settings will be used.
time in seconds
Time-to-live for artifact mappings, to limit the risk of artifact theft and misuse. After the elapsed time, mappings become invalid.
internal optimized in memory implementation
Optional reference to the id attribute of a previously defined <StorageService> element
When bound to a StorageService, an optional value can be supplied to isolate artifact mappings under a specific "context" key. Only needed if multiple deployments are sharing a single storage database.
Assuming a clustered deployment using an alternative <StorageService> element previously defined with an id of "MyMemcache":