logging: unify pkg/log and plugin/log (#2245)

Default to using pkg/log for all logging and use a fixed time prefix
which is RFC3339Millli (doesn't exist in time, so we just extended
RFC3339), i.e. Nano might be pushing it.

Logs go from:

2018/10/30 19:14:55 [INFO] CoreDNS-1.2.5
2018/10/30 19:14:55 [INFO] linux/amd64, go1.11,

to:

2018-10-30T19:10:07.547Z [INFO] CoreDNS-1.2.5
2018-10-30T19:10:07.547Z [INFO] linux/amd64, go1.11,

Which includes the timezone - which oddly the std log package doesn't
natively do.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/log/README.md

@@ -7,9 +7,10 @@
 ## Description
 
 By just using *log* you dump all queries (and parts for the reply) on standard output. Options exist
-to tweak the output a little.
+to tweak the output a little. The date/time prefix on log lines is RFC3339 formatted with
+milliseconds.
 
-Note that for busy servers this will incur a performance hit.
+Note that for busy servers logging will incur a performance hit.
 
 ## Syntax
 
@@ -59,7 +60,6 @@ The following place holders are supported:
 * `{name}`: qname of the request
 * `{class}`: qclass of the request
 * `{proto}`: protocol used (tcp or udp)
-* `{when}`: time of the query
 * `{remote}`: client's IP address, for IPv6 addresses these are enclosed in brackets: `[::1]`
 * `{size}`: request size in bytes
 * `{port}`: client's port
@@ -76,9 +76,15 @@ The following place holders are supported:
 The default Common Log Format is:
 
 ~~~ txt
-`{remote}:{port} - [{when}] {>id} "{type} {class} {name} {proto} {size} {>do} {>bufsize}" {rcode} {>rflags} {rsize} {duration}`
+`{remote}:{port} - {>id} "{type} {class} {name} {proto} {size} {>do} {>bufsize}" {rcode} {>rflags} {rsize} {duration}`
 ~~~
 
+Each of these logs will be outputted with `log.Infof`, so a typical example looks like this:
+
+~~~ txt
+2018-10-30T19:10:07.547Z [INFO] [::1]:50759 - 29008 "A IN example.org. udp 41 false 4096" NOERROR qr,rd,ra,ad 68 0.037990251s
+~~~~
+
 ## Examples
 
 Log all requests to stdout