allow user to set timeout parameters for tcp connections

[yomimono]

Frequently the user wants TCP.create_connection to fail quickly, or to try more persistently. The user has no control over this with the current API.

[yomimono]

Hearing no objections, I'll implement this.

[hannesm]

I'm wondering what the use case is exactly: are there different kind of TCP connections that need special treatment (i.e. a per-flow option), or should it be a global configuration option for the entire TCP stack (i.e. be able to set number of retransmissions and time delta between them) - which may depend on the location of the unikernel and its communication habits -- i.e. if it mostly does local connections, the time delta could be lowered?