Add connection logging to help with debugging

[wrosenuance]

While debugging connection problems it can be helpful to look at the TDS traffic, ideally the unencrypted traffic if encryption is enabled. This patch adds a connection logger that emits the encrypted and unencrypted packets as a hex dump, and also supports the TLS_LOG_KEY_FILE needed for TLS interception in tools like Wireshark.

Split out of #547 where it was first used/relevant.

[wrosenuance]

Hi @kardianos, this is the second piece of #547 - a self-contained patch around adding in connection logging.

[kardianos]

Great to see it, but I want to wait until the first one is settled a bit more.

[codecov]

Codecov Report

Merging #626 (768b5e3) into master (045585d) will decrease coverage by 0.05%.
The diff coverage is 78.68%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
- Coverage   71.98%   71.92%   -0.06%     
==========================================
  Files          24       25       +1     
  Lines        5469     5529      +60     
==========================================
+ Hits         3937     3977      +40     
- Misses       1309     1322      +13     
- Partials      223      230       +7     

Impacted Files	Coverage Δ
conn_str.go	99.18% <25.00%> (-0.82%)	⬇️
tds.go	66.54% <33.33%> (-1.06%)	⬇️
log_conn.go	100.00% <100.00%> (ø)
token.go	60.28% <0.00%> (-0.64%)	⬇️

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