Migrated svnexternals.txt file operations to a separate class.

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
[FIXED JENKINS-15098] - Added synchronization of all reads/modifications of svnexternals.txt file.

Resolves https://issues.jenkins-ci.org/browse/JENKINS-15098

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
[JENKINS-15098] - Refactoring
Moved base class to another package, modified visibility and javadoc.

Related issues: https://issues.jenkins-ci.org/browse/JENKINS-15098

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Removed CacheItem, added javax annotations to SvnExternalsFileManager

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Merge pull request #65 from synopsys-arc-oss/externalsfile-conflicts

[FIXED JENKINS-15098] - Access conflicts to svnexternals.txt