lwipopts: tcp parameter tuning for performance

Increased the following tcp parameters:

TCP Max segment size: 1024 -> 1460.
TCP send buffer size: 2048 -> 61320 (MSS * 42).
TCP recv buffer size: 8096 -> 61320.

Adjusted other parameters accordingly.
Especially, increased pre-allocated heap size to 1000000.

Change-Id: Idd71fbe349762a80df6c34c98978c371afa5523e
1 file changed