RFR [8054221] StringJoiner imlementation optimization

[Ivan Gerasimov]

Hello!

Here's the proposal by Martin Buchholz to improve implementation of 
StringJoiner class:
http://cr.openjdk.java.net/~martin/webrevs/openjdk9/StringJoiner-optimization/

I think that result of concatenation in the merge() method can be saved 
to be reused later.
This would be useful, for example, if a StringJoiner is merged into 
other StringJoiners several times.

So, here's my slightly modified version of the Martin's webrev:
http://cr.openjdk.java.net/~igerasim/8054221/0/webrev/

I've also modified the test to regularly test cases with empty 
prefixes/suffixes.

Sincerely yours,
Ivan