vlib: mitigate outdated new cli session events

vlib: mitigate outdated new cli session events

Possible races while concurrent additon to the new sessions vector
in a one process and remove from it in an another need to be avoided.
Let the vector be changed in the new session process function only.
Also cli_file_pool element may be freed already at the new session
event arrive timepoint, still causing unexpected cli banner for
noninteracive cli sessions.

Type: fix
Fixes: 17a67218587d40541ff522c6a86f354720481fbb
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: I022d16dd3aad9c9330834d35c58938f04b015b08