MM-21012: Revamp websocket implementation (#16620)

* MM-21012: Revamp websocket implementation

We replace the old gorilla/websocket implementation with the
gobwas/ws library. The gorilla library was in maintenance mode
and had a high level API due to which we cannot use that for
situations where a large number of concurrent connections needs
to be supported.

The ws library is a very low-level library that allows us
to work with raw net.Conns. We make several improvements:

- We completely remove the reader goroutines, and instead
replace them with a manual epoll implementation which sends off
messages to be read when it receives any data on the connection.
This lets us scale to a much larger number of connections.
- The reader buffer is eliminated, because we directly read
from the connection now.

https://mattermost.atlassian.net/browse/MM-21012

```release-notes
Improved the websocket implementation by using epoll manually
to read from a websocket. As a result, the number of goroutines
is expected to go down by half.
```

* fix tests

* fix shadowing errors

* final changes

* windows support!

* Remove pointer to waitgroup

* Fix edge case

* Trigger CI

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

vendor/github.com/gobwas/pool/internal/pmath/pmath.go

@@ -0,0 +1,65 @@
+package pmath
+
+const (
+	bitsize       = 32 << (^uint(0) >> 63)
+	maxint        = int(1<<(bitsize-1) - 1)
+	maxintHeadBit = 1 << (bitsize - 2)
+)
+
+// LogarithmicRange iterates from ceiled to power of two min to max,
+// calling cb on each iteration.
+func LogarithmicRange(min, max int, cb func(int)) {
+	if min == 0 {
+		min = 1
+	}
+	for n := CeilToPowerOfTwo(min); n <= max; n <<= 1 {
+		cb(n)
+	}
+}
+
+// IsPowerOfTwo reports whether given integer is a power of two.
+func IsPowerOfTwo(n int) bool {
+	return n&(n-1) == 0
+}
+
+// Identity is identity.
+func Identity(n int) int {
+	return n
+}
+
+// CeilToPowerOfTwo returns the least power of two integer value greater than
+// or equal to n.
+func CeilToPowerOfTwo(n int) int {
+	if n&maxintHeadBit != 0 && n > maxintHeadBit {
+		panic("argument is too large")
+	}
+	if n <= 2 {
+		return n
+	}
+	n--
+	n = fillBits(n)
+	n++
+	return n
+}
+
+// FloorToPowerOfTwo returns the greatest power of two integer value less than
+// or equal to n.
+func FloorToPowerOfTwo(n int) int {
+	if n <= 2 {
+		return n
+	}
+	n = fillBits(n)
+	n >>= 1
+	n++
+	return n
+}
+
+func fillBits(n int) int {
+	n |= n >> 1
+	n |= n >> 2
+	n |= n >> 4
+	n |= n >> 8
+	n |= n >> 16
+	n |= n >> 32
+	return n
+}