Optimize SRTP/SRTCP encryption/decryption

[logist322]

Optimization of encryption and decryption of SRTP and SRTCP packets. Also adds ability to use OpenSLL for these purposes, that makes those processes much faster.

---

	BEFORE	NO OPENSSL	WITH OPENSSL
RTP encrypt	1.7006 µs	1.2812 µs	1.0104 µs
RTP decrypt	1.5056 µs	1.2890 µs	1.0157 µs
RTCP encrypt	476.51 ns	284.18 ns	220.90 ns
RTCP decrypt	470.61 ns	319.27 ns	265.27 ns

[codecov]

Codecov Report

Patch coverage: 80.00% and project coverage change: -0.03% ⚠️

Comparison is base (dd340dd) 61.61% compared to head (9833c77) 61.59%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
- Coverage   61.61%   61.59%   -0.03%     
==========================================
  Files         541      542       +1     
  Lines       49903    49914      +11     
  Branches    12406    12394      -12     
==========================================
- Hits        30748    30743       -5     
- Misses       9938     9943       +5     
- Partials     9217     9228      +11     

Files Changed	Coverage Δ
srtp/src/error.rs	0.00% <ø> (ø)
srtp/src/cipher/cipher_aes_cm_hmac_sha1/mod.rs	65.71% <65.71%> (ø)
srtp/src/context/srtcp.rs	76.00% <80.00%> (+13.93%)	⬆️
...rc/cipher/cipher_aes_cm_hmac_sha1/opensslcipher.rs	81.45% <81.45%> (ø)
srtp/src/context/srtp.rs	85.71% <82.35%> (+17.29%)	⬆️
srtp/src/cipher/cipher_aead_aes_gcm.rs	84.68% <100.00%> (+1.04%)	⬆️
srtp/src/context/context_test.rs	80.92% <100.00%> (+0.65%)	⬆️
srtp/src/key_derivation.rs	90.62% <100.00%> (+6.75%)	⬆️

... and 16 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[logist322]

@rainliu please take a look at this