Depricate ProfiledMongoServer constructors, setting for max instances (#83)

The constructor removed by
https://github.com/kalahari/MiniProfiler.NET/commit/8aec81aa294ccf41a1ee0d3068495dfa358acd45
is re-created, and both constructors are marked obsolete.  The hard
limit for ProfiledMongoServer instances is now in a configurable value.