Don't store or send side-chain blocks lower than last checkpoint.
Change new constants in txdb.h to int64_t

A shift overflow was happening when using these to check against in init.cpp.
Fixes #3702.
qt: Rename 'Confirmed' balance to 'Spendable' on overview page

The word 'Spendable' more precisely says what the balance actually means.

Avoids the confirmed/unconfirmed confusion that can be caused by #3676.
Merge pull request #3699

ddcabae qt: Rename 'Confirmed' balance to 'Spendable' on overview page (Wladimir J. van der Laan)
Merge pull request #3703

ad54a9b Change new constants in txdb.h to int64_t (Wladimir J. van der Laan)
Merge pull request #2910

d8b4b49 Don't store or send side-chain blocks lower than last checkpoint. (Ashley Holman)
Remove "conflicted" as transaction category.

We were losing information about sent/received by overriding the
category in case of a conflicted transaction.

Hence, remove the "conflicted" category.

Conflicted status of a transaction can still be determined by looking
for confirmations<0.
Merge pull request #3720

b96f6a7 Remove "conflicted" as transaction category. (Wladimir J. van der Laan)
Make the signrawtransaction docs explicit that the arguments can be 'null'.
Merge pull request #3731 from gmaxwell/signrawnulldocs

Make the signrawtransaction docs explicit that the arguments can be 'null'
Also switch the (unused) verification code to low-s instead of even-s.

a81cd968 introduced a malleability breaker for signatures
(using an even value for S). In e0e14e43 this was changed to
the lower of two potential values, rather than the even one.
Only the signing code was changed though, the (for now unused)
verification code wasn't adapted.