Document new `walletpassphrase` behavior in 0.9

Also add a note to the release notes.
Fixes #3672.
Serialize only CTransaction data in gettransaction RPC hex

Don't include trailing implementation-specific wallet metadata.
Fixes 3a1c20b.
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 #3690 from laanwj/2014_02_gettransaction_serialize_only_tx

Serialize only CTransaction data in gettransaction RPC hex
Merge pull request #3679 from laanwj/2014_02_walletpassphrase_behavior

Document new `walletpassphrase` behavior in 0.9
Fix crash in importwallet and dumpwallet formatting

- DecodeDumpTime was passing a statically allocated facet object to
  std::locale. However, "The constructed locale object takes over
  responsibility for deleting this facet object." causing a free()
  crash on scope exit. Fixes #3670.

- EncodeDumpTime was using the wrong format character for dates
  (appears accidentally introduced in 51ed9ec9)
Merge pull request #3691

a143d4c Fix crash in importwallet and dumpwallet formatting (Wladimir J. van der Laan)
Merge pull request #3687

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