WL#4162: Add start/end of trx/stm to binlog

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.