logd: Fix pruning

In a scenario in which an on-line (blocking) client is running and
a clean is attempted (logcat -c), the following can be observed:

1) the on-line logger seems to freeze
2) any other clear attempt will have no effect

What is actually happening:

In this case prune function will "instruct" the oldest timeEntry
to skip a huge number (very close to ULONG_MAX) of messages, this
being the cause of 1.

Since the consumer thread will skip all the log entries, mStart
updating will also be skipped. So a new cleaning attempt will have
the same oldest entry, nothing will be done.

Fix description:
a. keep a separated skipAhead count for individual log buffers (log_id_t)
      LogTimeEntry::LogTimeEntry
      LogTimeEntry::FilterSecondPass
      LogTimeEntry::skipAhead
      LogTimeEntry::riggerSkip_Locked

b. update LogTimeEntry::mStart even if the current message is skipped
      LogTimeEntry::FilterSecondPass

c. while pruning, only take into account the LogTimeEntrys that are monitoring
   the log_id in question, and provide a public method of checking this.
      LogTimeEntry::isWatching
      LogTimeEntry::FilterFirstPass
      LogTimeEntry::FilterSecondPass

d. Reset the skip cont befor the client thtread starts to sleep, at this point
   we should be up to date.
      LogTimeEntry::cleanSkip_Locked
      LogTimeEntry::threadStart

Change-Id: I1b369dc5b02476e633e52578266a644e37e188a5
Signed-off-by: TraianX Schiau <traianx.schiau@intel.com>