IDEA
====
Use single bits of the Common-Header of Log_Event to encode start and end of
transactions and statements.

RATIONALE
=========
1. To save space in binary log (don't need an entire event for
   begin/commit)
2. Easier to solve bugs such as BUG#26395 and BUG#29288.
3. We need a uniform way to determine statement/transaction start/end
   without having to parse the entire event. This paves the way for
   things like WL#3735 and WL#2540, makes debugging easier, and forces
   us to use a consistent model for the transactional behavior.

We want each of these event categories to be clearly expressed in

the binary log:



   1. Transaction start

   2. Statement start (this includes any event that is used 

      as "seed" for coming events)

   3. Inside statement

   4. Statement end

   5. Transaction commit/autocommitted statement

   6. Transaction rollback



   Probably need 2 start bits: 

   01. No_start

   10. Statement_start

   11. Transaction_start

   

   Probably need 2 end bits: 

   00. No_end

   01. Transaction_commit (or autocommit)

   10. Rollback

   11. Statement_end



Note that a replication slave can detect that the events come from

an old master by reading the start bits: if they are 00, it must be

an old master.



Transaction_start means that the current event is the first event in

a transaction (hence, also the first in a statement).

Transaction_commit and Rollback mean that the current statement is

the last in a transaction (hence also the last in a statement).



For transactions involving XA tables, the last event describing a

statement (usually Query_log_event or row event) has only the

Statement_end flag set, and the Transaction_commit flag

is written for the following XID event.

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