perf: store Tablet.replicas as tuple instead of list

Replicas are never mutated after Tablet construction; convert to tuple
in __init__ to save 8 bytes per tablet (list overallocates for future
appends that never happen) and communicate immutability.

Before: 328 bytes/tablet (replicas container: 80 bytes as list)
After:  320 bytes/tablet (replicas container: 72 bytes as tuple)
Saving: 8 bytes/tablet (2.4%)

Combined with __slots__ (commit 1), total savings so far: 96 bytes/tablet.

Scale impact (3 replicas/tablet):
  128,000 tablets: saves ~1.0 MB (tuple) + 10.7 MB (slots) = 11.7 MB total
  256,000 tablets: saves ~2.0 MB (tuple) + 21.5 MB (slots) = 23.5 MB total

Author: mykaul

cassandra/tablets.py

@@ -20,7 +20,7 @@ class Tablet(object):
     def __init__(self, first_token=0, last_token=0, replicas=None):
         self.first_token = first_token
         self.last_token = last_token
-        self.replicas = replicas
+        self.replicas = tuple(replicas) if replicas is not None else None
 
     def __str__(self):
         return "<Tablet: first_token=%s last_token=%s replicas=%s>" \