Implement WL#2661 "Prepared Statements: Dynamic SQL in Stored Procedures".

The idea of the patch is to separate statement processing logic,
such as parsing, validation of the parsed tree, execution and cleanup, 
from global query processing logic, such as logging, resetting
priorities of a thread, resetting stored procedure cache, resetting
thread count of errors and warnings.
This makes PREPARE and EXECUTE behave similarly to the rest of SQL
statements and allows their use in stored procedures.
This patch contains a change in behaviour:
until recently for each SQL prepared statement command, 2 queries
were written to the general log, e.g.
[Query]   prepare stmt from @stmt_text;
[Prepare] select * from t1 <-- contents of @stmt_text
The chagne was necessary to prevent [Prepare] commands from being written
to the general log when executing a stored procedure with Dynamic SQL.
We should consider whether the old behavior is preferrable and probably
restore it.
This patch refixes Bug#7115, Bug#10975 (partially), Bug#10605 (various bugs
in Dynamic SQL reported before it was disabled).

mysql-test/r/not_embedded_server.result

@@ -1,5 +1,5 @@
 prepare stmt1 from ' show full processlist ';
 execute stmt1;
 Id	User	Host	db	Command	Time	State	Info
-number	root	localhost	test	Execute	time	NULL	show full processlist
+number	root	localhost	test	Query	time	NULL	show full processlist
 deallocate prepare stmt1;