switch from sysctl to setsockopt for setting recv/send buffer sizes

[gregcusack]

Problem

receive and send buffer sizes should not be set system wide to 128MB.

Summary of Changes

1. Leave the system wide max buffer sizing
2. Remove system wide default buffer sizing
3. use secksockopt to set the send and receive buffer sizes
4. remove OS check for default, system wide send/recv buffer sizes

NOTES

1. This PR does not actually change any buffer sizes. This is just reimplementing the same buffer sizes at runtime instead of system wide via sysctl.
2. There will be follow up PR(s) to
   a) reduce the size of the send or recv buffer on the unused side of sockets
   b) make better sizing decisions on socket buffers based on actual use

[steviez]

There will be follow up PR(s) to ...

Obviously out of scope for this PR, but we might consider hidden CLI args (maybe non-hidden at some point) to control this as well

[t-nelson]

Obviously out of scope for this PR, but we might consider hidden CLI args (maybe non-hidden at some point) to control this as well

I'd probably punt cli args altogether. The next iteration will be to selectively set buffers on the ports that actually need it. We don't even offer the ability to configure service port number individually. If we intend to support precision service socket configurations, we should do that all in one change set with consideration for consistency

[t-nelson]

lgtm. thanks!

[behzadnouri]

I think some places in the code are bypassing this method; e.g.
https://github.com/anza-xyz/agave/blob/21d754000/core/src/banking_stage/forwarder.rs#L53
https://github.com/anza-xyz/agave/blob/21d754000/gossip/src/cluster_info.rs#L227
https://github.com/anza-xyz/agave/blob/21d754000/gossip/src/cluster_info.rs#L3005