logd: use coalesce instead of merge (cleanup)

- switch to coalesce instead of merge in naming of functions
  and variables. Confusing since we also to merge-sorts and
  other activities in the logger.
- define maxPrune rather than using a number in the code path.

Bug: 24511000
diff --git a/logd/LogStatistics.h b/logd/LogStatistics.h
index 5a44d59..6041ae6 100644
--- a/logd/LogStatistics.h
+++ b/logd/LogStatistics.h
@@ -321,7 +321,7 @@
     void subtract(LogBufferElement *entry);
     // entry->setDropped(1) must follow this call
     void drop(LogBufferElement *entry);
-    // Correct for merging two entries referencing dropped content
+    // Correct for coalescing two entries referencing dropped content
     void erase(LogBufferElement *e) { --mElements[e->getLogId()]; }
 
     std::unique_ptr<const UidEntry *[]> sort(size_t n, log_id i) { return uidTable[i].sort(n); }