perf: add __slots__ to Tablet to eliminate per-instance __dict__

Add __slots__ to the Tablet class, removing the per-instance __dict__
allocation. Tablets are created frequently (one per token range per table)
and are long-lived, so the cumulative memory savings are significant.

Before: 416 bytes/tablet (48 instance + 96 __dict__ + 80 replicas + 192 tuples)
After:  328 bytes/tablet (56 instance +  0 __dict__ + 80 replicas + 192 tuples)
Saving: 88 bytes/tablet (21%)

Scale impact (3 replicas/tablet):
  12,800 tablets (100 tables x 128): saves 1.1 MB
 128,000 tablets (1000 tables x 128): saves 10.7 MB
 256,000 tablets (1000 tables x 256): saves 21.5 MB

Tablet.from_row construction also improves:
  Before: 186 ns/call
  After:  147 ns/call (1.27x faster, -21%)

Author: mykaul

cassandra/tablets.py

@@ -15,9 +15,7 @@ class Tablet(object):
     It stores information about each replica, its host and shard,
     and the token interval in the format (first_token, last_token].
     """
-    first_token = 0
-    last_token = 0
-    replicas = None
+    __slots__ = ('first_token', 'last_token', 'replicas')
 
     def __init__(self, first_token=0, last_token=0, replicas=None):
         self.first_token = first_token