(trunk libT) more heap pruning: avoid four unnecessary malloc() + free() calls per...

(trunk libT) more heap pruning: avoid four unnecessary malloc() + free() calls per tr_peer.

This commit also changes tr_recentHistory from being a general-purpose tool to being a little more hardcoded for the only purpose it's used, in tr_peerMgr. If its files (history.[ch]) don't find any other "customers" in libtransmission, eventually it should be demoted to being a private helper class inside of peer-mgr.c and have the history.[ch] files removed from the build.