use accurate log10 for meter (fast_log2 is unsuitable inaccurate)

fast_coefficient_to_dB() returns a lower bound value, unsuitable
to catch audio peaks. The difference to 20*log10 is as large as 0.4 dB!

The effective speedup of fast_log10 compared to log10f is marginal
(sweep of all 24bit values)

  i686 (1.6GHz Intel core):  2.36 [times faster]
  x86_64 (core2 2.4GHz):     1.63
  x86_64 (I3 2.80GHz):       2.03

the execution time of one log10f() averaged over a
sweep of all 24 bit values

  i686 (1.6GHz Intel core):  0.131 usec
  x86_64 (core2 2.4GHz):     0.033 usec
  x86_64 (I3 2.80GHz):       0.044 usec

PeakMeter::run() is called from dedicated non-rt, no harm done.

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