Since read only transactions may throw TransactionFailedRetriableException, provide method to retry them

[carrino]

Also provide some doc that this retriable version should not be used for long running read only transactions.
<T, E extends Exception> T runTaskReadOnlyWithRetry(TransactionTask<T, E> task) throws E;

lockImmutableTsOnReadOnlyTransactions = true may cause read only txn to throw.

    @Override
    public <T, C extends PreCommitCondition, E extends Exception> T runTaskWithConditionReadOnly(
            C condition, ConditionAwareTransactionTask<T, C, E> task) throws E {
        if (transactionConfig.get().lockImmutableTsOnReadOnlyTransactions()) {
            return runTaskWithConditionThrowOnConflict(condition, task);
        } else {
            return runTaskWithConditionReadOnlyInternal(condition, task);
        }
    }

which will throw in SnapshotTransaction.throwIfImmutableTsOrCommitLocksExpired