[network] Add fragmented write support to fake sockets
[network] Add fragmented write support to fake sockets

+ The writeable fake sockets now support an option to enable fragmented
writes, which forces writes to happen one byte at a time. Like the
fragmented read option, this helps ensure our socket logic is properly
handling partial reads and writes.

+ Rename `fragmented` to `fragmented_read` to disambiguate from
`fragmented_write`.

+ Simplify fake sockets' reading logic

+ Explicitly panic on reading an empty buffer, since the behaviour is
underspecified and not portable.

+ Remove some `#[dead_code]` attributes that aren't necessary anymore.

libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core