Allow the Monitor to purge unused queries

This is done by running a query against the monitor's queryindex and collecting
all queries referenced, and then swapping this collection out for the previous
cache.

To be threadsafe, updates to the queryindex are now done while holding the
readlock of a ReadWriteLock, and the cache holds the writelock firstly during
the creation of an update log, and then when the update log is copied to the
new cache and the caches are swapped.

PURGE-NOTES

@@ -0,0 +1,68 @@
+indexer thread              purge thread
+
+                            clone tlog
+    add docs                                <-- dropped!
+
+                            collect
+
+    commit
+
+    clear tlog
+
+                            swap tlog
+
+-----------------------------------------
+
+* add docs + commit is guarded by a read lock
+* purge thread creates purge log
+* purge log creation guarded by write lock
+* if purge log exists, indexer adds its queries to it
+* purge thread adds purge log contents after collectiong
+
+
+
+    add docs         |_
+    add to purge log |      <-- no purge log, so dropped!
+
+                         -| create purge log
+
+                            collect
+
+    commit           
+                         _| copy purge log
+                          | swap cache      
+
+So I need to make adding docs to the cache and committing protected by the
+readlock.
+
+* update _parses_ the docs, but doesn't add them to the cache
+* commit() now takes a list of CacheEntry objects
+
+    add & parse docs
+                        -| create purge log
+
+                           collect
+
+                        _| copy purge log
+                         | swap cache
+
+    add to cache     |
+    add to purge log |-
+    commit           |
+
+
+* problem now is: what happens if another thread commits before the cache is
+  populated?  We can get NPEs from the cache
+    - silently drop?  We don't guarantee that the doc is there until update()
+      returns, so I think this is OK.
+
+    add & parse docs
+                            delete docs
+                            commit
+                                                match in index
+                                                retrieve from cache -> NPE!
+
+
+    add to cache     |
+    add to purge log |-
+    commit           |

src/main/java/uk/co/flax/luwak/MatchError.java

@@ -36,4 +36,9 @@ public MatchError(String queryId, Exception error) {
         this.queryId = queryId;
         this.error = error;
     }
+
+    @Override
+    public String toString() {
+        return queryId + ":" + error.getMessage();
+    }
 }