Prevent lookups from seeing a hosts file reload in progress - #430

HostsFileParser reloaded a changed file by clearing its maps in place and
refilling them, while lookups read the maps without the lock. A lookup in
that window found nothing, even for an entry present in both versions of
the file. The first read had the same window: the maps were published
before the file was parsed.

Each read of the file now builds its own cache, published with a single
volatile write and never cleared, and each lookup works on one snapshot.

This also resets the "entire file parsed" flag per read, so a file that
grows beyond dnsjava.hostsfile.max_size_bytes is searched again instead of
reporting every uncached name as absent.

Closes dnsjava#429

…lling thread state

The test paused the first read and slept in a loop until the concurrent
lookup showed as BLOCKED. It now holds the read on a latch, checks that no
cache has been published and that the concurrent lookup has not answered,
then releases the read and expects both lookups to find the entry. A
package-private hasCache(), next to cacheSize(), exposes the published
state.