wal_writer_inbox_size is useless because readahead can not be set up to match

wal_writer_inbox_size is a useless parameter -- that is, it's useless to set it to a large value.
The reason for that is that cfg.readhead default is only 16k, and it's impossible to change
it without a significant impact on memory consumption of client fibers -- the same configuration
parameter is used for all fibers.

Besides, if I am reading the code correctly, as long as write_to_disk never yields, at most
1 inbox is used anyway.

Suggested fix:
- use libeio.