ovsdb-server: Eliminating max session limit

ovsdb-server: Eliminating max session limit

This patch removes limits on number of concurrent sessions
allowed by ovsdb-server. Historically, it was not an design
goal for OVSDB server to support very high number of sessions.
The imposed limit reflects those design choices.

Work is now underway to improve OVSDB scalability since supporting
large of number of sessions is important for OVN, Removing
this limit makes scalability testing possible.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Han Zhou <zhouhan@gmail.com>
Acked-by: Ben Pfaff <blp@ovn.org>