Use parallel stream to process YANG text files

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>