platform: add GSO support

[camshaft]

This change adds GSO for Linux. It passes the segment size through the cmsg(3) interface. A couple things to note:

• I had to add the ability to dynamically disable GSO when getting an EIO error from the OS. I initially tried to probe just localhost but that led to a false positive, which made all of the interop tests fail.
• A few of the platform internals had to be changed to accommodate the ability to offload segments. The nice thing is from the transport crate's perspective, basically nothing changed!
• I ran into an issue with MTU probing where we wrote a packet with 1200 bytes and then we try to push another for the probe but the queue has clamped the payload to 1200 bytes since it has the same destination address. I got around this by adding a can_gso to the tx::Message trait that tells the IO layer if it can be used as part of a segment.
• I went through and converted all of the storage data structures to be pinned to make sure they have stable pointers. It's unlikely given our usage, but in case they get reallocated it would result in a bunch of dangling pointers.

Overall these changes produce some good results. Here's the before/after of CPU usage to saturate a 1GbE:

Before

After

Here's the flamegraphs for the same test:

Before

After

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