improved file size calculation and speeds

This reverts commit 3b33712d.

indexing the full path exeeds the maximum index length for MySQL

initial version of a local storage implementation which will use unique slugified filename on the local filesystem.
This implementation will only be enabled on windows based system to solve the issues around UTF-8 file names with php on windows.

remove the index on the share table because of problems with the index size. Thanks to icewind for spotting this.

add a few more indexes. This is just a first step. More work is needed here but this should improve perfomance already for big installations.