MEDIUM: checks: Associate a session to each tcp-check healthcheck

MEDIUM: checks: Associate a session to each tcp-check healthcheck

Create a session for each healthcheck relying on a tcp-check ruleset. When such
check is started, a session is allocated, which will be freed when the check
finishes. A dummy static frontend is used to create these sessions. This will be
useful to support variables and sample expression. This will also be used,
later, by HTTP healthchecks to rely on HTTP muxes.