Prevent wallet from rescanning beyond prune depth

[pinheadmz]

This PR replaces #532 and is built on top of #749.

Once #749 is merged, the wallet will be able to determine whether the node it's connected to is pruned (whether its running as a plugin or standalone service). The prune depth is hard coded in networks.js for each network, so a wallet can prevent the user from rescanning more blocks than are available on disk.

Currently in bcoin, the rescanning process starts with a walletDB rollback (effectively "unconfirming" all the transactions in the DB from the rescan height to the tip), and THEN it scans the blockchain from the given height. If the given rescan height is lower than tip height - prune depth, those blocks will never be scanned and therefore the wallet will be out of sync with the state of the full blockchain.

This PR will throw if the user has requested a rescan beyond the available blocks on disk. The check is added to walletDB but is also added to wallet http -- the reason the http check is needed in addition is because the http call returns success: true before rescanning begins (since in practice its a background process).

Finally, to make testing more practical the regtest pruning settings have been lowered significantly. Added to the changelog is this message:

• The pruning configuration has been changed for regtest network: The new
  settings will start pruning after height 500, and the node will keep 288
  blocks. The previous settings were 1000 and 10000, respectively.

[codecov-io]

Codecov Report

Merging #751 into master will increase coverage by 0.64%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
+ Coverage   56.64%   57.28%   +0.64%     
==========================================
  Files         112      115       +3     
  Lines       28140    28356     +216     
  Branches     4800     4811      +11     
==========================================
+ Hits        15939    16244     +305     
+ Misses      12201    12112      -89

Impacted Files	Coverage Δ
lib/protocol/networks.js	100% <ø> (ø)	⬆️
lib/node/http.js	58.04% <ø> (+10.05%)	⬆️
lib/wallet/nodeclient.js	85.29% <100%> (+3.47%)	⬆️
lib/wallet/http.js	51.71% <100%> (+3.19%)	⬆️
lib/wallet/client.js	81.81% <100%> (ø)
lib/wallet/walletdb.js	77.72% <100%> (+0.42%)	⬆️
lib/wallet/nullclient.js	82.5% <100%> (+0.92%)	⬆️
lib/mempool/fees.js	52.5% <0%> (-8.84%)	⬇️
lib/node/spvnode.js	54.71% <0%> (ø)
lib/wallet/node.js	100% <0%> (ø)
... and 13 more

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