Data: Clear more caches when under memory pressure and at shutdown

core/java/src/net/i2p/data/Hash.java

core/java/src/net/i2p/data/PublicKey.java

core/java/src/net/i2p/data/SDSCache.java

core/java/src/net/i2p/data/SigningPublicKey.java

history.txt

router/java/src/net/i2p/router/Router.java