jsonrpc-server: Disconnect connections that queue too much data.

jsonrpc-server: Disconnect connections that queue too much data.

Consider this situation:

    * OVSDB client A executes transactions very quickly for a long time.

    * OVSDB client B monitors the tables that A modifies, but (either
      because B is connected over a slow network, or because B is slow to
      process updates) cannot keep up.

In this situation, the data that ovsdb-server has queued to send B grows
without bound and eventually ovsdb-server runs out of memory.  This commit
avoids the problem by noticing that more data is queued to B than necessary
to express the whole contents of the database and dropping the connection
to B.  When B reconnects later, it can then fetch the contents of the
database using less data than was previously queued to it.

(This is not entirely hypothetical.  We have seen this behavior in
intentional stress tests.)

Bug #15637.
Reported-by: Jeff Merrick <jmerrick@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>