Refactor our 'suspend operation' logic on bufferevents.

Refactor our 'suspend operation' logic on bufferevents.

There are lots of things we do internally in bufferevents to indicate
"the user would like this operation to happen, but we aren't going to
try until some other condition goes away."  Our logic here has gotten
entirely too complicated.

This patch tries to fix that by adding the idea of 'suspend flags' for
read and write.  To say "don't bother reading or writing until
condition X no longer holds," bufferevent_suspend_read/write(bev,
BEV_SUSPEND_X).  When X no longer holds, call
bufferevent_unsuspend_read/write(bev, BEV_SUSPEND_X).

Right now, only the read-watermark logic uses this.