a function to store information about transactions into buffers,
is added to the transaction manager, and called by the Checkpoint module.


storage/maria/ma_checkpoint.c:
  "collecting info about transactions" moves to trnman.c
storage/maria/trnman.c:
  a function to store information about the active transactions list
  and committed transactions list, into buffers, for use by the
  Checkpoint module.
  This function needs to know how many trns there are in the committed
  list, so we introduce a counter, trnman_committed_transactions.
  m_string.h is needed for LEX_STRING.
storage/maria/trnman.h:
  A function to store information about the active transactions list
  and committed transactions list, into buffers, for use by the
  Checkpoint module.
storage/maria/unittest/trnman-t.c:
  trnman.h needs LEX_STRING so m_string.h