txm: introduce transaction isolation levels

Now memtx TX manager tries to determine the best isolation level
by itself. There could be two options:
* READ_COMMITTED, when the transaction see changes of other tx
that are committed but not yet confirmed (written to WAL)
* READ_CONFIRMED, when the transaction see only confirmed changes.

Introduce a simple way to specify the isolation level explicitly:
box.begin{tx_isolation = 'default'} - the same as box.begin().
box.begin{tx_isolation = 'read-committed'} - READ_COMMITTED.
box.begin{tx_isolation = 'read-confirmed'} - READ_CONFIRMED.
box.begin{tx_isolation = 'best-effort'} - old automatic way.

Intrduce a bit more complex but potentially faster way to set
isolation level, like that:
my_level = box.tnx_isolation_level.READ_COMMITTED
..
box.begin{tx_isolation = my_level}

For simplicity of implementation also support symmetric values as
'READ_COMMITTED' and box.tnx_isolation_level['read-committed'].

Introduce a new box.cfg option - default_tx_isolation, that is
used as a default when a transaction is started. The option is
dynamic and possible values are the same as in box.begin, except
'default' which is meaningless.

In addition to string value the corresponding numeric values can
be used in both box.begin and box.cfg.

Part of #6930
NO_DOC=see later commits
NO_CHANGELOG=see later commits