mssql: de-couple the packet size from the buffer size

[kardianos]

If the packet size is set too small, the read buffer size will
overflow. Prevent this by setting the buffer size independent of the
packet size.

For #166

[codecov]

Codecov Report

Merging #304 into master will increase coverage by 0.09%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
+ Coverage   66.77%   66.86%   +0.09%     
==========================================
  Files          22       22              
  Lines        4695     4690       -5     
==========================================
+ Hits         3135     3136       +1     
+ Misses       1348     1343       -5     
+ Partials      212      211       -1

Impacted Files	Coverage Δ
tds.go	65.5% <100%> (ø)	⬆️
mssql.go	83.5% <100%> (+0.23%)	⬆️
buf.go	86.75% <96.96%> (+2.7%)	⬆️

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