Keep sessions around between each ConanRequester #287

Job	Run time
Setup and Cache Dependencies	1m 56s
Conan (unittests) (3.8)	1m 23s
Conan (integration) (3.8)	4m 7s
Conan (functional) (3.8)	26m 51s
Conan (unittests) (3.12)	42s
Conan (integration) (3.12)	2m 19s
Conan (functional) (3.12)	25m 58s
Conan (unittests) (3.13)	40s
Conan (integration) (3.13)	1m 46s
Conan (functional) (3.13)	18m 24s
	1h 24m 6s

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