BUG/MINOR: guid/server: ensure thread-safety on GUID insert/delete

BUG/MINOR: guid/server: ensure thread-safety on GUID insert/delete

Since 3.0, it is possible to assign a GUID to proxies, listeners and
servers. These objects are stored in a global tree guid_tree.

Proxies and listeners are static. However, servers may be added or
deleted at runtime, which imply that guid_tree must be protected. Fix
this by declaring a read-write lock to protect tree access.

For now, only guid_insert() and guid_remove() are protected using a
write lock. Outside of these, GUID tree is not accessed at runtime. If
server CLI commands are extended to support GUID as server identifier,
lookup operation should be extended with a read lock protection.

Note that during stat-file preloading, GUID tree is accessed for lookup.
However, as it is performed on startup which is single threaded, there
is no need for lock here. A BUG_ON() has been added to ensure this
precondition remains true.

This bug could caused a segfault when using dynamic servers with GUID.
However, it was never reproduced for now.

This must be backported up to 3.0. To avoid a conflict issue, the
previous cleanup patch can be merged before it.