Separate documents by a unique sequence and make partition backwards scannable

[albe]

This exchanges the \n (newline) document separator with the sequence \x00\x00\x1E\n, which is distinguishing enough to be searchable in the partition. Hence, this will allow checking for unfinished writes and search the last valid document and even scan through the partition backwards.
For optimization of that use case, the document data size is stored again before the document separator.

Resolves #124
Related to #31 #107

[coveralls]

Coverage increased (+0.2%) to 97.426% when pulling e1f5f5e on record-separator into d9c28c0 on master.

[albe]

t.b.d. - should the dataSize be added to the "footer", so that scanning backwards is more efficient?

[ghost]

DeepCode's analysis on #e1f5f5 found:

• ℹ️ 1 minor issue. 👇

Top issues

Description	Example fixes
Performing subtraction on a number (from number literal 0) and a boolean (from boolean literal false) will trigger an implicit coercion of the operands to number. If this is intended, consider making the coercion explicit. Occurrences: ReadablePartition.js:248 ReadablePartition.js:267 ReadablePartition.js:270	🔧 Example fixes

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