[Review] IGNITE-4648: IgniteInternalTx.prepare() does not wait for async operations to complete

[Дмитрий Рябов]

Hello, igniters, please review.

JIRA: https://issues.apache.org/jira/browse/IGNITE-4648

PR: https://github.com/apache/ignite/pull/1876/files

CI:
http://ci.ignite.apache.org/viewLog.html?buildId=590583&tab=buildResultsDiv&buildTypeId=IgniteTests_RunAllTemp