*: Add file meta cache to avoid resolve log time

[crazycs520]

When there are 1000 tidb.log files, the function resolveFiles will cost much time to read the file start time and the file end time. This PR add a meta cache to avoid resolve log file time.

Here is a benchmark when there are 1000 tidb.log files.

Before

search_log_test.go:670: searchLogSuite.BenchmarkResolveFiles            50          40212549 ns/op

This PR:

searchLogSuite.BenchmarkResolveFiles           100          18780837 ns/op

This cache has also been used in pingcap/tidb#21536.

[crazycs520]

/rebuild