Add freeze support to evbuffers.

Add freeze support to evbuffers.

From the documentation:
   Prevent calls that modify an evbuffer from succeeding. A buffer may
   frozen at the front, at the back, or at both the front and the back.

   If the front of a buffer is frozen, operations that drain data from
   the front of the buffer, or that prepend data to the buffer, will
   fail until it is unfrozen.   If the back a buffer is frozen, operations
   that append data from the buffer will fail until it is unfrozen.

We'll use this to ensure correctness on an evbuffer when we're waiting
for an overlapped IO call to finish.

svn:r1143