Use MaybeUninit more properly in streamer::batch_send()

[steviez]

Problem

batch_send() has undefined behavior. Some uninitialized data is marked as initialized with assume_init() even though it hasn't actually been initialized:

agave/streamer/src/sendmmsg.rs

Lines 138 to 140 in 10bebfa

	#[allow(clippy::uninit_assumed_init)]
	let iovec = std::mem::MaybeUninit::<iovec>::uninit();
	let mut iovs = vec![unsafe { iovec.assume_init() }; size];

This is problematic since the iovec struct has a pointer:

#[repr(C)]
pub struct iovec {
    pub iov_base: *mut c_void,
    pub iov_len: size_t,
}

With rust 1.82, this made the validator start crashing:

kernel: traps: solRspndrGossip[1514252] trap invalid opcode ip:561a11d83bb7 sp:7eb14c3a4830 error:0
sol.service: Main process exited, code=killed, status=4/ILL

Summary of Changes

• Use MaybeUninit interface to initialize iovec's
• Use MaybeUninit interface to initialize mmsghdr's
• Write every field in mmsghdr so that we no longer need to request zeroed data
• Use MaybeUninit interface to initialize sockaddr_storage's

[steviez]

Two things as I review this real quick:

• Should probably refactor addrs and hdrs to use MaybeUninit API as well
• recv_mmsg() has similar issues as batch_send(); I prefer smaller PR's so am initially inclined to do that separately but TBD