use 64Mb for on disk DOM cache for crengine
use 64Mb for on disk DOM cache for crengine

Previously we had cache size limit of 100 bytes, so crengine would erase
all on-disk cache on each file open as described in #206

NiLuJe/koreader-base,apletnev/koreader,Hzj-jie/koreader-base,apletnev/koreader-base,Hzj-jie/koreader-base,Frenzie/koreader-base,poire-z/koreader,Frenzie/koreader-base,noname007/koreader,Markismus/koreader,koreader/koreader-base,pazos/koreader,frankyifei/koreader-base,lgeek/koreader,apletnev/koreader-base,mwoz123/koreader,apletnev/koreader-base,robert00s/koreader,ashang/koreader,houqp/koreader-base,Hzj-jie/koreader,houqp/koreader-base,Frenzie/koreader-base,NiLuJe/koreader,apletnev/koreader-base,NiLuJe/koreader-base,koreader/koreader,poire-z/koreader,Frenzie/koreader,Hzj-jie/koreader-base,houqp/koreader-base,Hzj-jie/koreader-base,frankyifei/koreader-base,NiLuJe/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader-base,mihailim/koreader,frankyifei/koreader-base,houqp/koreader-base,houqp/koreader,Frenzie/koreader-base,NickSavage/koreader,chihyang/koreader,koreader/koreader-base,frankyifei/koreader-base,ashhher3/koreader,frankyifei/koreader,koreader/koreader-base,chrox/koreader,koreader/koreader-base,NiLuJe/koreader-base