Use speedy XOR from pion/transport

Now that we have a speedy version of xorBytes in pion/transport,
we might as well use it.  Benchmarks indicate an improvement
of 10% to 20% in CTR encryption speed for large blocks, depending
on the architecture.

Also remove TestXorBytesBuffer, which is no longer relevant.

Fix SRTCP packet length check

Packet length check of DecryptRTCP was wrong on AEAD profiles.

Avoid buffer allocation in Write

Use a sync.Pool for buffers used by (*SessionRTP).Write
and friends.  The pool is global, so its cost can be
amortised across multiple sessions.

Use MarshalTo in GCM encryptRTP

Use MarshalTo to place the header directly in the destination
buffer.

Optimise away calls to cipher.NewCTR

This avoids the allocation of a 512-byte buffer on each packet,
which leads to a 30% to 50% speedup in the SRTP benchmaks.

```
name                        old time/op    new time/op     delta
EncryptRTP-8                  1.58µs ± 8%     1.06µs ± 2%   -33.13%
EncryptRTPInPlace-8           1.54µs ± 3%     1.00µs ± 1%   -34.82%
DecryptRTP-8                  1.44µs ± 6%     0.67µs ± 2%   -53.80%
Write-8                       1.65µs ±10%     1.08µs ± 1%   -34.60%
WriteRTP-8                    1.52µs ± 2%     1.01µs ± 3%   -33.75%

name                        old speed      new speed       delta
EncryptRTP-8                71.1MB/s ± 8%  106.1MB/s ± 2%   +49.24%
EncryptRTPInPlace-8         72.9MB/s ± 3%  111.7MB/s ± 1%   +53.26%
DecryptRTP-8                19.4MB/s ± 6%   42.0MB/s ± 2%  +116.24%
Write-8                     68.2MB/s ± 9%  103.9MB/s ± 1%   +52.42%
WriteRTP-8                  73.6MB/s ± 2%  111.0MB/s ± 3%   +50.94%

name                        old alloc/op   new alloc/op    delta
EncryptRTP-8                    884B ± 0%       292B ± 0%   -66.97%
EncryptRTPInPlace-8             756B ± 0%       164B ± 0%   -78.31%
DecryptRTP-8                    780B ± 0%       188B ± 0%   -75.90%
Write-8                         916B ± 0%       324B ± 0%   -64.63%
WriteRTP-8                      804B ± 0%       212B ± 0%   -73.63%

name                        old allocs/op  new allocs/op   delta
EncryptRTP-8                    9.00 ± 0%       7.00 ± 0%   -22.22%
EncryptRTPInPlace-8             8.00 ± 0%       6.00 ± 0%   -25.00%
DecryptRTP-8                    9.00 ± 0%       7.00 ± 0%   -22.22%
Write-8                         9.00 ± 0%       7.00 ± 0%   -22.22%
WriteRTP-8                      8.00 ± 0%       6.00 ± 0%   -25.00%
```

Fix broken merge

d6ae2a5 missed RTP
API changes

Revert "Update github.com/pion/rtp to v2"

pion/webrtc is going to use /v0 until we are ready for a new major
version

Fix RolloverCount calculation error

Fix Unorder interval exceeds maxROCDisorder causing
RolloverCount error.

Fix destination buffer handling of AEAD cipher

- Fix not to change input buffer on decrypt
- Support in-place encrypt/decrypt

Fix buffer type assertion

Fix buffer type assertion to set read dead line on
packetio Buffer