[beta][tcp-scaling] Use INP_INFO_RLOCK in critical path, and use INP_INFO_WLOCK in full INP loops.

Julien's review:
 o Fix INP_INFO_{W,R}LOCK assertions
 o TIME_WAIT changes:
  - Check tw->cred != NULL
  - Fix a tw leak in tcp_tw_2msl_reuse()
  - tcp_tw_2msl_reuse() simplification
  - Deprecate using of tw reference counting