Overwriting small file with big file breaks syncing / access to file

### Step 1: Please describe your environment

* ZeroNet version: latest py3
* Operating system: Debian

### Step 2: Describe the problem:

Replacing a file which was fitting "small file limit" with a file that doesn't leads to file being unavailable on other nodes (at least on proxies). No `.piecemap.msgpack` is generated, even after deleting the file, signing changes and bringing it back; so i suspect that's the cause of the problem on remote nodes.

#### Steps to reproduce:

1. Add small optional file (lets say "file.bin") to a site and sign changes
2. Replace small "file.bin" with a big "file.bin" and sign changes
3. Try to access "file.bin" from other nodes

Proxies result in either time out or not found error

Reproduce the three listed steps on ZeroNet latest py3 under Debian, then compare the small-to-large replacement with a normal large-file addition. Start by tracing how file replacement, signing changes, and .piecemap.msgpack generation interact; done means the replaced file is available from other nodes and proxies without timeouts or not-found errors.

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