Improve fastboot verbose output.

Move some verbose messages so they're only output with -v.

Remove some misleading verbose messages altogether (such as "wiping
userdata...").

Properly log all the commands sent and responses received. Example:

  $ fastboot -v flashall
  ...
  Sending sparse 'system' 2/2 (443712 KB)
  fastboot: verbose: sending command "download:1b1500d0"
  fastboot: verbose: received DATA 1b1500d0
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (165596160 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (267762688 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (20978688 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (7168 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (208 bytes)
  fastboot: verbose: received OKAY
  OKAY [ 13.871s]
  Writing sparse 'system' 2/2
  fastboot: verbose: sending command "flash:system"
  fastboot: verbose: received OKAY
  OKAY [  3.228s]
  Rebooting
  fastboot: verbose: sending command "reboot"
  fastboot: verbose: received OKAY

  Finished. Total time: 36.939s

Bug: http://b/30953083
Bug: http://b/74444116
Test: `fastboot -v flashall`
Change-Id: I2cca198daa062fd1fb732218343263803b111e38