MINOR: spoe: Add "maxconnrate" and "maxerrrate" statements

MINOR: spoe: Add "maxconnrate" and "maxerrrate" statements

"maxconnrate" is the maximum number of connections per second. The SPOE will
stop to open new connections if the maximum is reached and will wait to acquire
an existing one.

"maxerrrate" is the maximum number of errors per second. The SPOE will stop its
processing if the maximum is reached.

These options replace hardcoded macros MAX_NEW_SPOE_APPLETS and
MAX_NEW_SPOE_APPLET_ERRS. We use it to limit SPOE activity, especially when
servers are down..