* dgud/mnesia/checkpoint-crash/OTP-14841:
  mnesia: Fix checkpoint crash

* dgud/mnesia/slow-startup/OTP-14829:
  mnesia: Read schema user properties directly

Bad timing can cause retain messages to go to a new process if
checkpoint name is reused directly and the checkpoints contain
different tables.

Ignore those messages instead of crash.

Doing a proper transaction handling of user properties (in current
implmenentation) caused schema_transactions to be quadratic which
caused huge startup times for nodes with many tables since the merge schema
is a schema_transaction.

Do a direct read instead, cheap but implies that the plugin changes
will not be visible until the changes are commited and cannot be used
later in same transaction, which was possible before if undocumented
functions where used.

* dgud/mnesia/backup-error-handling/OTP-14776:
  mnesia: Fix error handling in abort write

The create_option() type in mnesia lacks the user_properties field
which is used by mnesia in various places.

The "root cause" error reason was lost when abort_write failed,
e.g. file was already closed, and Mod:abort_write(..) returned an
error.