Speed up debug logging, when filtered out

[bboreham]

A benchmark, plus two changes:

• Filter by log level before anything else - this saves a lot of work walking the stack to find the caller, and a bit more to format the timestamp. As recommended at Filtered-out log.Level lines still cost go-kit/log#14 (comment)
• Avoid Sprintf when log line is filtered out - defer the Sprintf until after the level check by using an auxilliary struct.

Benchmark results:

name      old time/op    new time/op    delta
Debugf-4    2.46µs ± 6%    0.54µs ± 6%  -78.00%  (p=0.008 n=5+5)

name      old alloc/op   new alloc/op   delta
Debugf-4      848B ± 0%      328B ± 0%  -61.32%  (p=0.008 n=5+5)

name      old allocs/op  new allocs/op  delta
Debugf-4      15.0 ± 0%       9.0 ± 0%  -40.00%  (p=0.008 n=5+5)

Small improvement in behaviour for Infoln(), Warningln(), etc.: they no longer add a newline character to the string. Go-kit log lines are intrinsically a single line, with structured fields so it's wrong to add a newline.
Previously this would come out as msg="My log message\n" in the log.

[pracucci]

Smart, good job!