perf(transport): remove Server::timers

The current `neqo_transport::server::Server::timers` has a large performance
overhead, especially when serving small amount of connections. See
#1780 for details.

This commit optimizes for the small-number-of-connections case, keeping a single
callback timestamp only, iterating each connection when there is no other work
to be done.

neqo-common/Cargo.toml

@@ -34,7 +34,3 @@ features = ["timeapi"]
 [lib]
 # See https://github.com/bheisler/criterion.rs/blob/master/book/src/faq.md#cargo-bench-gives-unrecognized-option-errors-for-valid-command-line-options
 bench = false
-
-[[bench]]
-name = "timer"
-harness = false

neqo-common/src/lib.rs

@@ -14,7 +14,6 @@ pub mod hrtime;
 mod incrdecoder;
 pub mod log;
 pub mod qlog;
-pub mod timer;
 pub mod tos;
 
 use std::fmt::Write;