Decrease MAX_BACKENDS to 2^16

Hi,

Currently the maximum for max_connections (+ bgworkers + autovacuum) is
defined by
#define MAX_BACKENDS 0x7fffff
which unfortunately means that some things like buffer reference counts
need a full integer to store references.

Since there's absolutely no sensible scenario for setting
max_connections that high, I'd like to change the limit to 2^16, so we
can use a uint16 in BufferDesc->refcount.

Does anyone disagree? This clearly is 9.5 material, but I wanted to
raise it early, since I plan to develop some stuff for 9.5 that'd depend
on lowering it.

Greetings,

Andres Freund

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Andres Freund http://www.2ndQuadrant.com/
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