jsonrpc: Treat receiving part of a message as activity.

jsonrpc: Treat receiving part of a message as activity.

Until now, the jsonrpc code has only counted receiving a full JSON-RPC
messages as activity.  This could theoretically time out, then, while a
very long message is in transit or if a slow link is involved.  This commit
changes this code to count receiving any part of a message as activity.

This isn't a problem for OpenFlow connections because OpenFlow messages are
at most 64 kB in size.

This problem hasn't actually been observed in practice.

Bug #12789.
Signed-off-by: Ben Pfaff <blp@nicira.com>