IGNITE-4188: Savepoints support inside of Ignite Transactions, thoughts about realisation

[Dmitrii Ryabov]

Hello, community. Sam, Yakov, Alex G., can you look my decision?

try (Transaction tx = ignite.transactions().txStart()) {
    c.put(1, 1);

    tx.savepoint("mysavepoint");

    c.put(2, 2);

    tx.rollbackToSavepoint("mysavepoint");

    c.put(3, 3);

    tx.commit();
}


I want to make it in such way:

When user calls tx.savepoint(""), Transaction delegate call to
IgniteInternalTx, which delegate it to its IgniteTxState.

IgniteTxState implementation will contain stack of savepoints, where
Savepoint is class containing snapshot of cache entries and any important
additional information.

What do you think?