Add PQGO_DEBUG to print server communication to stderr

[arp242]

This can be pretty useful to debug protocol issues and see what's going on. For example:

% PQGO_DEBUG=1 go test -run TestSimpleQuery
CLIENT → Startup                 69  "\x00\x03\x00\x00database\x00pqgo\x00user [..]"
SERVER ← (R) AuthRequest          4  "\x00\x00\x00\x00"
SERVER ← (S) ParamStatus         19  "in_hot_standby\x00off\x00"
[..]
SERVER ← (Z) ReadyForQuery        1  "I"
         START conn.query
         START conn.simpleQuery
CLIENT → (Q) Query                9  "select 1\x00"
SERVER ← (T) RowDescription      29  "\x00\x01?column?\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x17\x00\x04\xff\xff\xff\xff\x00\x00"
SERVER ← (D) DataRow              7  "\x00\x01\x00\x00\x00\x011"
         END conn.simpleQuery
         END conn.query
SERVER ← (C) CommandComplete      9  "SELECT 1\x00"
SERVER ← (Z) ReadyForQuery        1  "I"
CLIENT → (X) Terminate            0  ""
PASS
ok      github.com/lib/pq       0.010s

This works everywhere and is checked once on startup, so that people can do things like:

PQGO_DEBUG=1 my-app

Which can be useful for some bug reports.

Also move some of this connection string parsing stuff from conn.go to connector.go. Makes rather more sense to put it there.