NBN_Connection layout is cache unfriendly

[Phireh]

__NBN_Connection is big enough (a little above 1 MB per connection) that it doesn't fit into the L2 cache of many systems.

A quick look into a frequently called function like Connection_ReadNextMessageFromStream shows that channels and endpoint are accessed often and immediately after each other, however, their positions in the struct are very far apart. On my Linux 64b system they look like this:

(gdb) p offsetof(NBN_Connection, channels)
$1 = (NBN_Channel *(*)[32]) 0x107318

(gdb) p offsetof(NBN_Connection, endpoint)
$2 = (struct __NBN_Endpoint **) 0x30

More than a MB apart. I believe this is introducing cache misses and impacting performance. The fix is very straightforward: move frequently used members of the struct closer together.

[nathhB]

@Phireh hey, do you want to do this one?

[nathhB]

Merged this PR: #26