commitlog: Use disk file alignment info (with lower value if possible)
commitlog: Use disk file alignment info (with lower value if possible)

Previously, the disk block alignment of segments was hardcoded (due to
really old code). Now we use the value as declared in the actual file
opened. If we are using a previously written file (i.e. o_dsync), we
can even use the sometimes smaller "read" alignment.

Also allow config to completely override this with a disk alignment
config option (not exposed to global config yet, but can be).

v2:
* Use overwrite alignment if doing only overwrite
* Ensure to adjust actual alignment if/when doing file wrapping

v3:
* Kill alignment config param. Useless and unsafe.

Closes #8935

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla