e3: Appendable type

[AskAlexSharov]

No description provided.

[awskii]

need close out here

[AskAlexSharov]

no. can't just close file - even if you just merged it.
because there is still can be alive readers (which see/read this file). Example of "reader": RPCDaemon's roTx.
amount of readers can get by out.refcount.Load().
so, the right handling of merged files must be:

// if merged file not visible for any alive reader (even for us): can remove it immediately
// otherwise: mark it as `canDelete=true` and last reader of this file - will remove it inside `aggRoTx.Close()`
if out.refcount.Load() == 0 { 
   out.closeFilesAndRemove()
} else { 
    out.canDelete.Store(true)
}

created helper func for it. and used everywhere.