[NXCM-5957] Add merge request strategy that will check upfront if merged metadata is stale

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NXCM-5957] Register the merge metadata strategy with group repository when yum merge capability is enabled on a group repository

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NXCM-5957] Touch repomd.xml after is generated so sha1 attribute is calculated

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NXCM-5957] Store member repository repomd.xml sha1's after merging

It also adds proper read locking of member repos repomd's and write lock on merged repomd

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NXCM-5957] Change IT to not force merging

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>