feat(dup): implement procedure load_from_private_log

[neverchanje]

load_from_private_log performs the heaviest procedure among the duplication:
loading mutation-logs from disk.

The entire process:

1. load_from_private_log::run:
   ensure the logs from start_decree are available on disk.

2. load_from_private_log::find_log_file_to_start:
   select the log file to load.

3. load_from_private_log::replay_log_block:
   loading in a block by block way. once EOF is hit, switch to the next file.

4. Once a log block is loaded, the mutations will be passed down to the next stage, aka ship_mutations.

The unit test of this procedure is instrumented by some testing methods:

1. write and load: write a number of fake mutations to disk and load. The next stage after load_from_private_log is mocked as verification of the loaded logs.

2. handle real-world logs, not only fake logs: given files that were generated from a real pegasus server.

3. fault injection: the replaying of logs and file opening are injected with faults in 25% probability. This helps us to test the corner cases.