test: rename serveronly tests to reflect the new extension name

evolve: stop running 'serveronly' setup Since there is no common logic remaining the smaller extension we can stop running its setup in the main extensions

discovery: directly merge the 'obsdiscovery' exthelper in top level extensions There is no reason to proxy it through the 'obsexchange' submodule.

compat: drop advertising of unsupported experimental bundle2 part The support for this part was dropped long ago in 74bc8a0c2c02. But we forgot to remove it from the 'simple4server' extension.

exchange: dispatch all code in 'serveronly' to the appropriate submodule All code related to exchange is now in either 'evolve.obsexchange' or 'evolve.obsdiscovery', and the 'serveronly' extension is using their 'exthelper' to set itself up. As a side effect, the 'serveronly' extensions now enable better exchange when pushing from that server too.

capabilities: properly sort capabilities after updating them Capabilities are usually sorted. This is especially useful to avoid silly error in test.

pull: also issue warning for old mercurial version

exchange: split between modern and legacy code We make it clearer what code is old crust and what code is the cutting edge part.

push: add extra warning about pushing to old server Pushing using old method is slow and racy. We adds warning to point this to the user. We also add inline comment to clarify the function purpose.

push: exit obsexchange early if disabled We were exiting the function later, in particular -after- issuing a message about exchanging obsmarkers.