[WIP] histroy: Adjust cache size OnMemoryStateChange()

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index 5b8d39964ab2aa4e96312b790606c7b0bd0c8b29..9cab2b3dcd72b48647b486eedd4f7f175e1fb39b 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -288,6 +288,11 @@ class SQL_EXPORT Connection {
   // usage by half.
   void TrimMemory(bool aggressively);
 
+  // Update the cache size. Unlike set_cache_size(), this will have an effect
+  // even after initialization.  Returns true if cache size is successfully
+  // updated.  The total cache size in bytes will be page_size * new_cache_size.
+  bool UpdateCacheSize(int new_cache_size);

[Scott Hess - ex-Googler, 2016/09/01 18:29:09]

I want to complain that this should be called Set rather than Update, but, sigh,
set_cache_size() would be confusing.

Maybe it should be SetCacheSize(), and if the database was not open yet it would
set cache_size_ and exit, but if the database was open it would also issue the
pragma.

[Maybe the pre-init configuration should really be pushed to a
Connection::Config object passed to the constructor.  I'll think about that.]

+
   // Raze the database to the ground.  This approximates creating a
   // fresh database from scratch, within the constraints of SQLite's
   // locking protocol (locks and open handles can make doing this with