JDBC batch updates

[nicktorwald]

Add support for JDBC batch updates. It includes an implementation of
Statement.*Batch(...) as well as PreparedStatement.*Batch() methods.

Under the hood SQLConnection uses the pipelining sending requests one by
one asynchronously and awaiting all of them. There are some issues
regarding vinyl storage engine where execution order are not specified
and DDL statements which are not transactional.

Closes: #62

[coveralls]

Coverage increased (+0.9%) to 73.228% when pulling 05bfde6 on nicktorwald/gh-62-batch-updates into b53e0ba on master.

[Totktonada]

LGTM.

The only comment. It would be good to mention guarantees we provide in README: I mean, say that batching guarantees order of execution for memtx engine, but does not for vinyl.

[nicktorwald]

I didn't that, 'cause I thought It's worth describing in a separate document which is dedicated to JDBC driver user manual, isn't it?
README is ok, but too many JDBC things are required to be mentioned, so README can become too large for reading. Don't you think so?

[Totktonada]

I don't want to miss such details. While we don't sure how the whole documentation will be structured and hosted, I think it is okay to dump such things to README. Another possible way to don't miss them is to file an issue with 'documentation' label, but this looks as an overcomplicated process for me.

So I propose to charge README with everything a user may want (as part of development process) and postpone structural questions for now. Once we'll define a structure, we'll split README to appropriate documents.