Remove synchronization from TargetStorage that prevents better parallelization

[aecio]

Remove synchronization from TargetStorage that prevents some CPU-bound tasks such as language detection and page classification to run concurrently.

[aecio]

A very simple benchmark on my machine (CPU with 32 cores) shows following improvements:

(Crawl execution was timed using Linux's time tool.)

BEFORE (commit f47f2dd):

real    15m0.426s
user    34m1.744s
sys 1m7.540s

Crawl time: ~15 minutes
User/real time: 2.26
Observed CPU usage: around 200%-300%
Crawled pages: 4.314
pages/min: 287.6

AFTER (commit 6025a26):

real    20m35.727s
user    232m24.804s
sys 3m3.800s

Crawl time: ~20 minutes
User/real time: 11.6
Observed CPU usage: around 1200%-1900%
Crawled pages: 65.240
pages/min: 3106.67

Considering the pages/min metric, it is a 980% improvement. There's still room for improvement, since there are still some big portions of code that are still synchronized for thread safety.