Store: cemented metadata cannot exceed 4Gib

Cemented files are the place holders were cemented blocks (blocks not subject to reorgs) are stored, as chunks (usually conforming to the definition of protocol cycles). The metadata are store in the same way, be they are compressed to gain disk space. camlzip is used to compress the data allowing to gain a decent disk space and allowing good performances as access to the data can be performed in a random way, without uncompressing the whole file.

Due to the 32 bits limitations of camlzip, and the fact that the 64bits version is likely not to exist https://github.com/xavierleroy/camlzip/issues/35 such zip files cannot:

• handle more that 2^16 - 1 entries
• handle files bigger that 4Gib

The first item was fixed by !12262 (merged)

We should make sure that the 4Gib limit won't be reached.