Improve performance of TCP socket reads

[mpilquist]

• Initializes the socket read buffer to 8192 bytes instead of 0
• Uses allocateDirect
• Stores read buffer in a private var instead of a Ref

[djspiewak]

Almost no difference.

Before

Throughput

Ramped up to 500 connections.
Total data sent:     16886.5 MiB (17706826192 bytes)
Total data received: 16885.3 MiB (17705547200 bytes)
Bandwidth per channel: 18.886⇅ Mbps (2360.8 kBps)
Aggregate bandwidth: 4721.438↓, 4721.779↑ Mbps
Packet rate estimate: 437386.1↓, 437942.3↑ (6↓, 8↑ TCP MSS/op)

Requests per Second

200 clients, running 128 bytes, 30 sec.

Speed: 77217 request/sec, 77213 response/sec
Requests: 2316522
Responses: 2316394

After

Throughput

Ramped up to 500 connections.
Total data sent:     17368.3 MiB (18212022044 bytes)
Total data received: 17370.2 MiB (18213932796 bytes)
Bandwidth per channel: 19.424⇅ Mbps (2428.0 kBps)
Aggregate bandwidth: 4856.241↓, 4855.732↑ Mbps
Packet rate estimate: 448966.6↓, 451067.4↑ (6↓, 8↑ TCP MSS/op)

Requests per Second

200 clients, running 128 bytes, 30 sec.

Speed: 78482 request/sec, 78478 response/sec
Requests: 2354488
Responses: 2354342