The binlog_cache_use is incremented twice when changes to a transactional table
are committed, i.e. TC_LOG_BINLOG::log_xid calls is called. The problem happens
because log_xid calls both binlog_flush_stmt_cache and binlog_flush_trx_cache
without checking if such caches are empty thus unintentionally increasing the
binlog_cache_use value twice.

Although not explicitly mentioned in the bug, the binlog_cache_disk_use presents
the same problem.

The binlog_cache_use and binlog_cache_disk_use are status variables that are
incremented when transactional (i.e. trx-cache) or non-transactional (i.e.
stmt-cache) changes are committed. They are used to compute the ratio between
the use of disk and memory while gathering updates for a transaction.

The problem reported is fixed by avoiding incrementing the binlog_cache_use
and binlog_cache_disk_use if a cache is empty. We also have decided to increment
both variables when a cache is truncated as the cache is used although its
content is discarded and is not written to the binary log.

In this patch, we take the opportunity to re-organize the code around the
function binlog_flush_trx_cache and binlog_flush_stmt_cache.

mysql-test/extra/binlog_tests/binlog_cache_stat.test:
  Updated the test case with comments and additional tests to check both
  transactional and non-transactional changes and what happens when a 
  is transaction either committed or aborted.
mysql-test/suite/binlog/r/binlog_innodb.result:
  Updated the result file.
mysql-test/suite/binlog/r/binlog_mixed_cache_stat.result:
  Updated the result file.
mysql-test/suite/binlog/r/binlog_row_cache_stat.result:
  Updated the result file.
mysql-test/suite/binlog/r/binlog_stm_cache_stat.result:
  Updated the result file.
mysql-test/suite/binlog/t/binlog_mixed_cache_stat.test:
  Updated the test case with a new source file.
mysql-test/suite/binlog/t/binlog_row_cache_stat.test:
  Updated the test case with a new source file.
mysql-test/suite/binlog/t/binlog_stm_cache_stat.test:
  Updated the test case with a new source file.
sql/log.cc:
  There are four changes in here:
  
    . Computed statistics on binlog_cache_use and binlog_cache_disk_use while
    resting the cache.
  
    . Refactored the code so binlog_flush_cache handles both the trx-cache and
    stmt-cache.
  
    . There are functions that encapsulate the calls to binlog_flush_cache and
    make easier to read the code.
  
    . binlog_commit_flush_stmt_cache is now taking into account the result:
    success or error.
sql/log_event.h:
  Guaranteed Xid_log_event is always classified as a transactional event.