core/rawdb: set freezerdb.readOnly to prevent Freeze deadlock in read-only mode

[MozirDmitriy]

• Assign readOnly: opts.ReadOnly in Open() when constructing freezerdb
• Without this, calling freezerdb.Freeze() in read-only mode attempts to send on chainFreezer.trigger while the freezer loop isn’t running, causing a permanent block.
• The change enforces the intended behavior where Freeze() returns errReadOnly under read-only operation, avoiding deadlock and aligning with the function’s guard.