wireproto: provide accessors for client capabilities
For HTTP, this refactors the existing logic, including the parsing of
the compression engine capability.
For SSH, this adds a ssh-only capability "protocaps" and a command for
informing the server on what the client supports. Since SSH is stateful,
keep track of the capabilities in the server instance.
Differential Revision: https://phab.mercurial-scm.org/D1944
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t