ovsdb: Raise the jsonrpc server session limit

Raise the connection limit to allow larger number of concurrent
ovsdb-server connections. Note, ovsdb-server may not perform well
at the new limit. It is rather a prelude to further scaling tests and
optimizations.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>