DOC: server/threads: document which functions need to be called with/without locks

DOC: server/threads: document which functions need to be called with/without locks

At the moment it's totally unclear while reading the server's code which
functions require to be called with the server lock held and which ones
grab it and cannot be called this way. This commit simply inventories
all of them to indicate what is detected depending on how these functions
use the struct server. Only functions used at runtime were checked, those
dedicated to config parsing were skipped. Doing so already has uncovered
a few bugs on some CLI actions.