Add support for Git's v2 wire protocol

Git 2.18 introduces version 2 of the wire protocol used to service hosting operations. The new wire protocol offers more flexibility and some significant performance improvements over the original protocol, especially for repositories which have a large number of refs. Bitbucket Server should be updated to support the v2 wire protocol while still automatically falling back on v1 (which is likely to continue being the most common protocol for some time).

(Some further information on the v2 protocol is available here for those who are curious.)