Use parallel stream to process YANG text files
When we are examining local YANG files, we are also performing
first step of parsing to determine what the module/revision is
without relying on the file name. This process can take quite
some time if the models are large and/or there are many of them.
Since each file is independent at this stage, use a parallel
stream (and hence common FJ pool) to perform this task in multiple
threads before moving on to processing.
JIRA: YANGTOOLS-1062
Change-Id: I31cee2aa7f313052565ba723cf4c52566821a625
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>