Rename CircPriorityHalflifeMsec to CircuitPriorityHalflifeMsec

Everything that accepted the 'Circ' name handled it wrong, so even now
that we fixed the handling of the parameter, we wouldn't be able to
set it without making all the 0.2.2.7..0.2.2.10 relays act wonky.
This patch makes Tors accept the 'Circuit' name instead, so we can
turn on circuit priorities without confusing the versions that treated
the 'Circ' name as occasion to act weird.

changes/read_circpriorityhalflife_correctly

@@ -2,4 +2,6 @@
    - Fix a stupid parenthesization error that made every possible value
      of CircPriorityHalflifeMsec get treated as "1 msec".  Bugfix on
      0.2.2.7-alpha.
-
+   - Rename CircPriorityHalflifeMsec to CircuitPriorityHalflifeMsec,
+     so authorities can tell newer relays about the option without
+     breaking older ones.

doc/spec/dir-spec.txt

@@ -1149,9 +1149,11 @@
         transit in the network at any given time. Obeyed by Tor 0.2.1.20
         and later.
 
-        "CircPriorityHalflifeMsec" -- the halflife parameter used when
+        "CircuitPriorityHalflifeMsec" -- the halflife parameter used when
         weighting which circuit will send the next cell. Obeyed by Tor
-        0.2.2.7-alpha and later.
+        0.2.2.10-alpha and later.  (Versions of Tor between 0.2.2.7-alpha
+        and 0.2.2.10-alpha recognized a "CircPriorityHalflifeMsec" parameter,
+        but mishandled it badly.)
 
    The authority section of a vote contains the following items, followed
    in turn by the authority's current key certificate:

src/or/relay.c

@@ -1865,9 +1865,9 @@ cell_ewma_set_scale_factor(or_options_t *options, networkstatus_t *consensus)
     source = "CircuitPriorityHalflife in configuration";
   } else if (consensus &&
              (halflife_ms = networkstatus_get_param(
-                   consensus, "CircPriorityHalflifeMsec", -1)) >= 0) {
+                   consensus, "CircuitPriorityHalflifeMsec", -1)) >= 0) {
     halflife = ((double)halflife_ms)/1000.0;
-    source = "CircPriorityHalflifeMsec in consensus";
+    source = "CircuitPriorityHalflifeMsec in consensus";
   } else {
     halflife = EWMA_DEFAULT_HALFLIFE;
     source = "Default value";