Make Redis pool size configurable

The Redis pool uses a fixed maximum size of 8 defined by the basic implementation of JedisPoolConfig. At least the maxiumum size (and maybe other GenericObjectPoolConfig parameters) should be made configurable.
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The Redis connection pool size can be configured using the new nuxeo.conf options:
nuxeo.redis.maxTotal (default 16)
nuxeo.redis.maxIdle (default 8)