This implements the annotation API proposed in golang.org/cl/63274.

traceString is updated to protect the string map with trace.stringsLock
because the assumption that traceString is called by a single goroutine
(either at the beginning of tracing and at the end of tracing when
dumping all the symbols and function names) is no longer true.

traceString is used by the annotation apis (NewContext, StartSpan, Log)
to register frequently appearing strings (task and span names, and log
keys) after this change.

NewContext -> one or two records (EvString, EvUserTaskCreate)
end function -> one record (EvUserTaskEnd)
StartSpan -> one or two records (EvString, EvUserSpan)
span end function -> one or two records (EvString, EvUserSpan)
Log -> one or two records (EvString, EvUserLog)

EvUserLog record is of the typical record format written by traceEvent
except that it is followed by bytes that represents the value string.

In addition to runtime/trace change, this change includes
corresponding changes in internal/trace to parse the new record types.

Future work to improve efficiency:
  More efficient unique task id generation instead of atomic. (per-P
  counter).
  Instead of a centralized trace.stringsLock, consider using per-P
  string cache or something more efficient.

R=go1.11

Change-Id: Iec9276c6c51e5be441ccd52dec270f1e3b153970
Reviewed-on: https://go-review.googlesource.com/71690Reviewed-by: Austin Clements <austin@google.com>