Add options to specify the maximum number of TCP connections

[saza-ku]

The maximum number of TCP connections are now hard-coded.

We may need to enable to specify it by something like options.

To change the number, we need to tune these variables: #6

But we're not familiar with lwIP's parameters in lwipopts.h. We've arbitrarily determined them. The settings of Unikraft may be helpful as a reference.