Fix for BUG#12228: SP cache code:

* Cleanup SP Cache code, now SP Cache only deletes sp_head objects in 
  sp_cache_flush_obsolete() invalidates all pointers to routines in the cache.
* Use new SP Cache use contract in the code.

There is no test case because it doesn't seem to be possible to cause thread races to end
the same way they end in heavy-load test. This patch removes the crash in heavy test.

mysql-test/r/type_bit.result

@@ -34,7 +34,7 @@ select 0 + b'1111111111111111';
 select 0 + b'1000000000000001';
 0 + b'1000000000000001'
 32769
-drop table if exists t1;
+drop table if exists t1,t2;
 create table t1 (a bit(65));
 ERROR 42000: Column length too big for column 'a' (max = 64); use BLOB or TEXT instead
 create table t1 (a bit(0));

mysql-test/r/view.result

@@ -1880,6 +1880,8 @@ test.v5	check	error	View 'test.v5' references invalid table(s) or column(s) or f
 test.v6	check	status	OK
 drop view v1, v2, v3, v4, v5, v6;
 drop table t2;
+drop function if exists f1;
+drop function if exists f2;
 CREATE TABLE t1 (col1 time);
 CREATE TABLE t2 (col1 time);
 CREATE TABLE t3 (col1 time);

mysql-test/t/type_bit.test

@@ -16,7 +16,7 @@ select 0 + b'1111111111111111';
 select 0 + b'1000000000000001';
 
 --disable_warnings
-drop table if exists t1;
+drop table if exists t1,t2;
 --enable_warnings
 
 --error 1074

mysql-test/t/view.test

@@ -1711,6 +1711,10 @@ CHECK TABLE v1, v2, v3, v4, v5, v6;
 drop view v1, v2, v3, v4, v5, v6;
 drop table t2;
 
+--disable_warnings
+drop function if exists f1;
+drop function if exists f2;
+--enable_warnings
 CREATE TABLE t1 (col1 time);
 CREATE TABLE t2 (col1 time);
 CREATE TABLE t3 (col1 time);