Rm_client is derived from Pager_object. If the Pager_object is also
derived from Thread_base (which is the case for NOVA) then the
Rm_client object must be destructed without holding the rm_session_object
lock. The native platform specific Thread_base implementation has to take
care that all in-flight page handling requests are finished before
destruction. On NOVA it is done by doing an IPC to the pager thread.
(performed in Pager_object::dissolve() in base-nova). The
called thread than executes its operation until end which also requires
in some cases to take the rm_session_object lock.
Since _client_slab insertion/deletion also must be performed
synchronized but can't be protected by the rm_session_object lock
because of the described dead_lock situation, we have
to use a synchronized allocator object to perform insertion and
deletion of Rm_clients.
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