Increase default dbcache to 100 MiB
add constants for shared (GUI/core) -dbcache settings

- adds nDefaultDbCache, nMaxDbCache and nMinDbCache in txdb.h
Merge pull request #3687

82e9600 add constants for shared (GUI/core) -dbcache settings (Philip Kaufmann)
879b390 Increase default dbcache to 100 MiB (Pieter Wuille)