plainbox:session:resume: detect missing log files

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:session:resume: refactor helper decoder

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:session:resume: track location of resumed session in resume stack
plainbox:session:resume: add flag rewrite-log-pathnames

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:session:resume: add flag ignore-job-checksums

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>