Customized temporary file directory, unified storage of temporary files during the synchronization process

[wobure123]

Customized temporary file directory, unified storage of temporary files during the synchronization process

[calmh]

Duplicate of #2208 I think

[wobure123]

I check the source code, since atomic operation is needed, it maybe hard to change the directory of temp file. The bug I came across in synchnizing a webdav type folder is: for instance a new 50 MB test.txt is added in the other machine, soon ,Syncthing begins to work, a downloading is started, temp file .syncthing.test.temp is created and soon finished, since I use davfs2 mount webdav file , there is also a .syncthing .test.temp created in davfs2 cache diretory, everything is fine, But aside with the temp file,there is a Target file text.txt also in the davfs2 cache diretory, and..., it is cycling from 0 MB to 50MB, the network is taken over by it, and syncthing is stuck in downloading the file, even though,the total temp file is already in there. It is so weird....