appender: remove Sync bound from writer for NonBlocking (#2607)

## Motivation

`NonBlocking` from `tracing-appender` wraps a writer and requires that
writer to implement `Sync` (among other bounds). However it is not clear
why this bound is necessary in first place: this writer is sent to a
dedicated thread and never used directly concurrently.

#1934 demonstrates that it is a real problem for some people. Also at my
current work we hit this issue as well when a third-party writer did not
implement `Sync`. Our workaround was to wrap that writer in our own type
and manually implement `Send` and `Sync` for it. Needless to say that it
is more a hack than a proper solution.

## Solution

Remove `Sync` bound in relevant places. Yep, that simple. Probably
having it in first place was just an oversight.

Closes #1934

tracing-appender/src/lib.rs

@@ -190,7 +190,7 @@ pub(crate) mod sync;
 /// });
 /// # }
 /// ```
-pub fn non_blocking<T: Write + Send + Sync + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
+pub fn non_blocking<T: Write + Send + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
     NonBlocking::new(writer)
 }
 

tracing-appender/src/non_blocking.rs

@@ -146,11 +146,11 @@ impl NonBlocking {
     ///
     /// [default]: NonBlockingBuilder::default
     /// [builder]: NonBlockingBuilder
-    pub fn new<T: Write + Send + Sync + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
+    pub fn new<T: Write + Send + 'static>(writer: T) -> (NonBlocking, WorkerGuard) {
         NonBlockingBuilder::default().finish(writer)
     }
 
-    fn create<T: Write + Send + Sync + 'static>(
+    fn create<T: Write + Send + 'static>(
         writer: T,
         buffered_lines_limit: usize,
         is_lossy: bool,
@@ -221,7 +221,7 @@ impl NonBlockingBuilder {
     }
 
     /// Completes the builder, returning the configured `NonBlocking`.
-    pub fn finish<T: Write + Send + Sync + 'static>(self, writer: T) -> (NonBlocking, WorkerGuard) {
+    pub fn finish<T: Write + Send + 'static>(self, writer: T) -> (NonBlocking, WorkerGuard) {
         NonBlocking::create(
             writer,
             self.buffered_lines_limit,

tracing-appender/src/worker.rs

@@ -4,7 +4,7 @@ use std::fmt::Debug;
 use std::io::Write;
 use std::{io, thread};
 
-pub(crate) struct Worker<T: Write + Send + Sync + 'static> {
+pub(crate) struct Worker<T: Write + Send + 'static> {
     writer: T,
     receiver: Receiver<Msg>,
     shutdown: Receiver<()>,
@@ -18,7 +18,7 @@ pub(crate) enum WorkerState {
     Shutdown,
 }
 
-impl<T: Write + Send + Sync + 'static> Worker<T> {
+impl<T: Write + Send + 'static> Worker<T> {
     pub(crate) fn new(receiver: Receiver<Msg>, writer: T, shutdown: Receiver<()>) -> Worker<T> {
         Self {
             writer,