make FilePositionEntry thread-safe

[kazegusuri]

ref #1804 and #1963

fluent-plugin-protometheus calls FilePositionEntry#read_pos and FilePositionEntry#read_inode by using instance_variable_get to get current position and inode as metrics. I understand it is unsupported use.
FilePositionEntry's methods are not thread-safe because each methods share @file to read data from file. So race happens between in_tail plugin and fluent-plugin-protometheus plugin.
But now I found instance variable @pe of Fluent::Plugin::TailInput::TailWatcher is accessible by outside. So I think it is better to be thread-safe to use FilePositionEntry without considering my usecase.
How do you think?

[kazegusuri]

Oh, part of tests failed. Is is flaky?

[repeatedly]

Closed by other PR.