db: use a rwmutex to protect access to the pool
db: use a rwmutex to protect access to the pool

Related to #269.

ArxdSilva/tsuru,4eek/tsuru,cgvarela/tsuru,gciotta/tsuru,hcxiong/tsuru,ggarnier/tsuru,gwmoura/tsuru,tsuru/tsuru,mderaldo/tsuru,tsuru/tsuru,cezarsa/tsuru,RichardKnop/tsuru,gwmoura/tsuru,Endika/tsuru,guilhermebr/tsuru,mderaldo/tsuru,4eek/tsuru,firebitsbr/tsuru,hugoantunes/tsuru,hugoantunes/tsuru,Endika/tsuru,kgrvamsi/tsuru,guilhermebr/tsuru,hcxiong/tsuru,firebitsbr/tsuru,gciotta/tsuru,guilhermebr/tsuru,cezarsa/tsuru,RichardKnop/tsuru,gwmoura/tsuru,ggarnier/tsuru,ggarnier/tsuru,jblancett/tsuru,4eek/tsuru,cgvarela/tsuru,ArxdSilva/tsuru,tuxmonteiro/tsuru,cezarsa/tsuru,cgvarela/tsuru,mderaldo/tsuru,kgrvamsi/tsuru,ArxdSilva/tsuru,cezarsa/tsuru,hcxiong/tsuru,Endika/tsuru,guilhermebr/tsuru,jblancett/tsuru,ggarnier/tsuru,jblancett/tsuru,tuxmonteiro/tsuru,tsuru/tsuru,RichardKnop/tsuru,tsuru/tsuru,firebitsbr/tsuru,kgrvamsi/tsuru,tuxmonteiro/tsuru,ArxdSilva/tsuru,gciotta/tsuru