txm: expose transaction isolation in iproto

Introduce new option IPROTO_TXN_ISOLATION (0x59) in the body of
IPROTO_BEGIN request, so a user can set isolation level similar
to box.begin in lua.

The value must be one of the following integers:
enum txn_isolation_level {
	/** Take isolation level from global default level. */
	TXN_ISOLATION_DEFAULT,
	/** Allow to read committed, but not confirmed changes. */
	TXN_ISOLATION_READ_COMMITTED,
	/** Allow to read only confirmed changes. */
	TXN_ISOLATION_READ_CONFIRMED,
	/** Determine isolation level automatically. */
	TXN_ISOLATION_BEST_EFFORT,
};

Support the new option in net.box.

Part of #6930
NO_DOC=see later commits
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